Lolterwebs In Teh Coastz0r
what he said.cyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoekenFlaming_Maniac wrote:
And Comet is...?
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Enlarge
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Enlarge
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Enlarge
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
QFTcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoekenFlaming_Maniac wrote:
And Comet is...?
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar sstem. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
Enlarge
Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Enlarge
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
Enlarge
Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
I agreespawnofthemist wrote:
QFTcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar sstem. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Enlarge
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Enlarge
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
To meet Wikipedia's quality standards, this article or section may require cleanup.
Please discuss this issue on the talk page, or replace this tag with a more specific message. Editing help is available.
This article has been tagged since June 2006.
[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
Enlarge
Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Enlarge
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
+1
I agree moreTowelly wrote:
I agreespawnofthemist wrote:
QFTcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Enlarge
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Enlarge
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
Enlarge
Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
+1
+2
QFEjkohlc wrote:
QFEcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Enlarge
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Enlarge
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Enlarge
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
You have to learn the rules of the game and then you have to play better than anyone else.
Sticky?cyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoekenFlaming_Maniac wrote:
And Comet is...?
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
Enlarge
Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
QFF!spawnofthemist wrote:
QFTcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken']['error wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar sstem. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
To meet Wikipedia's quality standards, this article or section may require cleanup.
Please discuss this issue on the talk page, or replace this tag with a more specific message. Editing help is available.
This article has been tagged since June 2006.
[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
Enlarge
Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Enlarge
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
from wiki:
A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ices. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek word komē, meaning "hair of the head," Aristotle first used the derivation komētēs to depict comets as "stars with hair."
Contents
[show]
* 1 Physical characteristics
* 2 Orbital characteristics
* 3 Comet nomenclature
* 4 History of comet study
o 4.1 Early observations and thought
o 4.2 Orbital studies
o 4.3 Studies of physical characteristics
o 4.4 Debate over comet composition
* 5 Notable comets
o 5.1 Great comets
o 5.2 Unusual comets
o 5.3 Currently visible comets
* 6 See also
* 7 References
* 8 External links
[edit] Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence).[1] They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend 150 million km (1 Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.[2]
Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.[4]
[edit] Orbital characteristics
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
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Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
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Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[5]
[edit] Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke[6] and Comet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus (174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).
[edit] History of comet study
[edit] Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[11] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[12]
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated, observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[14]
[edit] Orbital studies
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
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The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.[15]
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.[14] In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.[15]
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[16]
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9.[17] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.[21] By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of April 2006, 175 comets have achieved this distinction, though several have since been destroyed or lost. In ephemerides, comets are often denoted by the symbol ☄.
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[edit] Studies of physical characteristics
Comets have highly elliptical orbits. Note the two distinct tails.
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Comets have highly elliptical orbits. Note the two distinct tails.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
—James Thomson, "The Seasons" (1730; 1748).
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[22] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.[23]
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
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Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
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Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
im really getting real good at this! ^^WilhelmSissener wrote:
QFF!spawnofthemist wrote:
QFTcyborg_ninja-117 wrote:
comet: http://images.google.nl/images?client=f … gen+zoeken
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[24] A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[25] This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
Enlarge
Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[26]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was the only spacecraft other then Deep Impact itself to view the comet.
[edit] Debate over comet composition
Comet Borrelly exhibits jets, yet is hot and dry.
Enlarge
Comet Borrelly exhibits jets, yet is hot and dry.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[27]
The recent Deep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.[1]
[edit] Notable comets
[edit] Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images webpage.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[28]
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
[edit] Unusual comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn.[29] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[30] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3.[31] Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit.[32]
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system.[33] However, it has been suggested that the likely object responsible for the Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of the Giordano Bruno (crater) on the Moon in 1178.
[edit] Currently visible comets
Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June 20, 2006, as of October 6 is visible with binoculars in the predawn sky in the constellation Canes Venatici. Perihelion was September 28.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
Good info.
more info about comet
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
Comet diagram
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.
As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.
Comet West
Comet West
Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.
Comet Animation
* Historical background of comets.
Views of Comets
Comet Neat Comet Neat
This image of comet C/2001 Q4 (NEAT) was taken at the WIYN 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Arizona, on May 7, 2004.
The image was captured with the Mosaic I camera, which has a one-square degree field of view, or about five times the size of the Moon. Even with this large field, only the comet's coma and the inner portion of its tail are visible. A small star cluster (C0736-105, or Melotte 72) is visible in the lower right of the image, between the head of the comet and the bright red star in the lower-right corner. (Courtesy NASA, NOAO, NSF, STScI)
Comet Kohoutek Comet Kohoutek
This color photograph of the comet Kohoutek was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona. They photographed the comet from the Catalina observatory with a 35mm camera on January 11, 1974. (Courtesy NASA)
Comet Hyakutake Comet Hyakutake
These Hubble Space Telescope images of comet Hyakutake were taken on March 25, 1996 when the comet passed at a distance of 9.3 million miles from Earth. These images focus on a very small region near the heart of the comet, the icy, solid nucleus and provide an exceptionally clear view of the near-nucleus region of the comet.
The left image is 2070 miles across (3340 km) and shows that most of the dust is being produced on the sunward-facing hemisphere of the comet. Also at upper left are three small pieces which have broken off the comet and are forming their own tails. Icy regions on the nucleus are activated as they rotate into sunlight, ejecting large amounts of dust in the jets that are faintly visible in this image. Sunlight striking this dust eventually turns it around and "blows" it into the tailward hemisphere.
The bottom-right image is an expanded view of the near-nucleus region and is only 470 miles (760 km) across. The nucleus is near the center of the frame, but the brightest area is probably the tip of the strongest dust jet rather than the nucleus itself. Presumably, the nucleus surface lies just below this bright jet. The top-right image shows pieces of the nucleus that apparently broke off. The image shows at least three separate objects that are probably made up of coarse-grained dust. Large fragments of the nucleus would not be accelerated into the tail, which appears to be the case in this image. (Credit: H. A. Weaver--Applied Research Corp., HST Comet Hyakutake Observing Team, and NASA)
Comet Hyakutake First X-Rays From Comet Hyakutake Discovered
This image shows the discovery of a strong X-ray radiation signal coming from comet Hyakutake. The image was made on March 27, 1996 using Germany's orbiting ROSAT satellite. The comet was near its closest approach to the Earth at a distance of less than 10 million miles when X-ray emmisions were first detected by ROSAT. The strength and rapid changes in intensity of the comet's X-ray emission both surprised and puzzled astronomers. "We had no clear expectation that comets shine in X-rays," said Dr. Michael J. Mumma of NASA's Goddard Space Flight Center, Greenbelt, MD. X-rays have never been found from a comet before, and scientists had optimistically predicted an intensity that turned out to be about 100 times weaker than the radiation actually detected by ROSAT. Strong changes in the brightness of the X-rays were another surprise. There were pronounced increases and decreases in the X-ray brightness from one ROSAT observation to another, typically over a time difference of a few hours.
Still another puzzle is the nature of the physical process that generates the X-rays, but the ROSAT image may contain clues to this process. In the image, the X-rays from the comet seem to come from a crescent-shaped region on the sunward side of Comet Hyakutake. One preliminary theory is that X-ray emission from the Sun was absorbed by a cloud of gaseous water molecules surrounding the nucleus of the comet, and then were re-emitted by the molecules in a process physicists call "fluorescence." According to this idea, the cloud is so thick that its sunward side absorbs nearly all the incoming solar X-rays, so that none reach the remainder of the cloud. This could explain why the cometary X-ray emission has the form of a crescent, rather than that of a sphere around the nucleus. A second possible explanation is that the X-rays are produced from the violent collision between the comet material and the supersonic "wind" of plasma and particles streaming away from the Sun.
1993a Mueller Comet 1993a Mueller
This is a CCD image of comet 1993a Mueller, taken on October 6, 1993 with a 288mm f/5.2 Schmidt-Cassegrain telescope. The comet has a coma diameter of 3' and a fan-shaped tail, up to 7' long. (Courtesy Erich Meyer and Herbert Raab, Austria)
Comet West Comet West (1975)
This photograph was taken by amateur astronomer John Loborde on March 9, 1976. This picture shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles. (Courtesy John Laborde)
Comet West Comet West (1975)
This image of comet West was taken by John Laborde at the Tierra Del Sol Observatory site in San Diego County. The exposure was 30 minutes with a 135 mm Nikon lens. (Courtesy John Laborde)
Comet Hale-Bopp Comet Hale-Bopp
These NASA Hubble Space Telescope pictures of comet Hale-Bopp show a remarkable "pinwheel" pattern and a blob of free-flying debris near the nucleus. The bright clump of light along the spiral (above the nucleus, which is near the center of the frame) may be a piece of the comet's icy crust that was ejected into space by a combination of ice evaporation and the comet's rotation, and which then disintegrated into a bright cloud of particles.
Although the "blob" is about 3.5 times fainter than the brightest portion at the nucleus, the lump appears brighter because it covers a larger area. The debris follows a spiral pattern outward because the solid nucleus is rotating like a lawn sprinkler, completing a single rotation about once per week.
Comet Hale-Bopp Comet Hale-Bopp
This image of comet Hale-Bopp was taken by John Laborde with his home designed and built, 8.8" f/3.7 Wright Schmidt Camera. The picture was taken at the Tierra Del Sol Observatory site in San Diego County with a 25 minute exposure on Kodak PPF400 film. (Courtesy John Laborde)
Ikeya-Seki Comet Ikeya-Seki
This image of comet Ikeya-Seki was taken by John Laborde in Poway, California just before dawn. The exposure was 15 minutes with a 55 mm Nikon lens. (Courtesy John Laborde)
Fuck me it took me about a minute just to scroll down to the bottom!
It really isn't funny anymore']['error wrote:
im really getting real good at this! ^^WilhelmSissener wrote:
QFF!spawnofthemist wrote:
QFT
more info about comet
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.
Comet diagram
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.
As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.
The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.
Comet West
Comet West
Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.
Comet Animation
* Historical background of comets.
Views of Comets
Comet Neat Comet Neat
This image of comet C/2001 Q4 (NEAT) was taken at the WIYN 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Arizona, on May 7, 2004.
The image was captured with the Mosaic I camera, which has a one-square degree field of view, or about five times the size of the Moon. Even with this large field, only the comet's coma and the inner portion of its tail are visible. A small star cluster (C0736-105, or Melotte 72) is visible in the lower right of the image, between the head of the comet and the bright red star in the lower-right corner. (Courtesy NASA, NOAO, NSF, STScI)
Comet Kohoutek Comet Kohoutek
This color photograph of the comet Kohoutek was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona. They photographed the comet from the Catalina observatory with a 35mm camera on January 11, 1974. (Courtesy NASA)
Comet Hyakutake Comet Hyakutake
These Hubble Space Telescope images of comet Hyakutake were taken on March 25, 1996 when the comet passed at a distance of 9.3 million miles from Earth. These images focus on a very small region near the heart of the comet, the icy, solid nucleus and provide an exceptionally clear view of the near-nucleus region of the comet.
The left image is 2070 miles across (3340 km) and shows that most of the dust is being produced on the sunward-facing hemisphere of the comet. Also at upper left are three small pieces which have broken off the comet and are forming their own tails. Icy regions on the nucleus are activated as they rotate into sunlight, ejecting large amounts of dust in the jets that are faintly visible in this image. Sunlight striking this dust eventually turns it around and "blows" it into the tailward hemisphere.
The bottom-right image is an expanded view of the near-nucleus region and is only 470 miles (760 km) across. The nucleus is near the center of the frame, but the brightest area is probably the tip of the strongest dust jet rather than the nucleus itself. Presumably, the nucleus surface lies just below this bright jet. The top-right image shows pieces of the nucleus that apparently broke off. The image shows at least three separate objects that are probably made up of coarse-grained dust. Large fragments of the nucleus would not be accelerated into the tail, which appears to be the case in this image. (Credit: H. A. Weaver--Applied Research Corp., HST Comet Hyakutake Observing Team, and NASA)
Comet Hyakutake First X-Rays From Comet Hyakutake Discovered
This image shows the discovery of a strong X-ray radiation signal coming from comet Hyakutake. The image was made on March 27, 1996 using Germany's orbiting ROSAT satellite. The comet was near its closest approach to the Earth at a distance of less than 10 million miles when X-ray emmisions were first detected by ROSAT. The strength and rapid changes in intensity of the comet's X-ray emission both surprised and puzzled astronomers. "We had no clear expectation that comets shine in X-rays," said Dr. Michael J. Mumma of NASA's Goddard Space Flight Center, Greenbelt, MD. X-rays have never been found from a comet before, and scientists had optimistically predicted an intensity that turned out to be about 100 times weaker than the radiation actually detected by ROSAT. Strong changes in the brightness of the X-rays were another surprise. There were pronounced increases and decreases in the X-ray brightness from one ROSAT observation to another, typically over a time difference of a few hours.
Still another puzzle is the nature of the physical process that generates the X-rays, but the ROSAT image may contain clues to this process. In the image, the X-rays from the comet seem to come from a crescent-shaped region on the sunward side of Comet Hyakutake. One preliminary theory is that X-ray emission from the Sun was absorbed by a cloud of gaseous water molecules surrounding the nucleus of the comet, and then were re-emitted by the molecules in a process physicists call "fluorescence." According to this idea, the cloud is so thick that its sunward side absorbs nearly all the incoming solar X-rays, so that none reach the remainder of the cloud. This could explain why the cometary X-ray emission has the form of a crescent, rather than that of a sphere around the nucleus. A second possible explanation is that the X-rays are produced from the violent collision between the comet material and the supersonic "wind" of plasma and particles streaming away from the Sun.
1993a Mueller Comet 1993a Mueller
This is a CCD image of comet 1993a Mueller, taken on October 6, 1993 with a 288mm f/5.2 Schmidt-Cassegrain telescope. The comet has a coma diameter of 3' and a fan-shaped tail, up to 7' long. (Courtesy Erich Meyer and Herbert Raab, Austria)
Comet West Comet West (1975)
This photograph was taken by amateur astronomer John Loborde on March 9, 1976. This picture shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles. (Courtesy John Laborde)
Comet West Comet West (1975)
This image of comet West was taken by John Laborde at the Tierra Del Sol Observatory site in San Diego County. The exposure was 30 minutes with a 135 mm Nikon lens. (Courtesy John Laborde)
Comet Hale-Bopp Comet Hale-Bopp
These NASA Hubble Space Telescope pictures of comet Hale-Bopp show a remarkable "pinwheel" pattern and a blob of free-flying debris near the nucleus. The bright clump of light along the spiral (above the nucleus, which is near the center of the frame) may be a piece of the comet's icy crust that was ejected into space by a combination of ice evaporation and the comet's rotation, and which then disintegrated into a bright cloud of particles.
Although the "blob" is about 3.5 times fainter than the brightest portion at the nucleus, the lump appears brighter because it covers a larger area. The debris follows a spiral pattern outward because the solid nucleus is rotating like a lawn sprinkler, completing a single rotation about once per week.
Comet Hale-Bopp Comet Hale-Bopp
This image of comet Hale-Bopp was taken by John Laborde with his home designed and built, 8.8" f/3.7 Wright Schmidt Camera. The picture was taken at the Tierra Del Sol Observatory site in San Diego County with a 25 minute exposure on Kodak PPF400 film. (Courtesy John Laborde)
Ikeya-Seki Comet Ikeya-Seki
This image of comet Ikeya-Seki was taken by John Laborde in Poway, California just before dawn. The exposure was 15 minutes with a 55 mm Nikon lens. (Courtesy John Laborde)
Last edited by Jenspm (2006-11-07 08:37:35)
Is it on Robin Park?scottomus0 wrote:
LOL thats right.
Just got a phone call from the manager. Start on saturday.
Glad really coz ive been stuck at home for the past month.
Yeyz.
Good luck dude
Have you worked in retail before.
I bet your amazed by the number of smelly customers, don't spend your money here go buy some soap and washing powder
Have you worked in retail before.
I bet your amazed by the number of smelly customers, don't spend your money here go buy some soap and washing powder
Lol it sure is. How do you know. were you from?The Magic Mullet wrote:
Is it on Robin Park?scottomus0 wrote:
LOL thats right.
Just got a phone call from the manager. Start on saturday.
Glad really coz ive been stuck at home for the past month.
Yeyz.
he's behind you right now.
Why do I get a feeling that temps are locked on target to this thread?
Last edited by Sarrk (2006-11-08 04:44:29)
Dont say that, im on my own.
Warrington, you scummy pie eaterscottomus0 wrote:
Lol it sure is. How do you know. were you from?The Magic Mullet wrote:
Is it on Robin Park?scottomus0 wrote:
LOL thats right.
Just got a phone call from the manager. Start on saturday.
Glad really coz ive been stuck at home for the past month.
Yeyz.
need to close this
next time make a post that has a bit more info scot
next time make a post that has a bit more info scot
