Comet Hale-Bopp, showing a white dust tail and blue gas tail (February 1997)
A comet is a small astronomical object similar to an asteroid but composed largely of ice. Comets typically move in highly elliptical orbits, the aphelia of which may be many times more distant than Pluto's orbit. Often described as "dirty snowballs", comets are composed largely of frozen carbon dioxide, methane and water with dust and various mineral aggregates mixed in.
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.
Comets are believed to originate in a distant cloud known as the Oort cloud, after the astronomer Jan Hendrik Oort who hypothesised its existence. They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that bring them very close to the Sun. When a comet approaches the inner solar system, radiation from the Sun causes its outer layers of ice to evaporate. The streams of dust and gas this releases form a huge but 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 pointing away from the sun. The dust and gas each form their own distinct tail, pointed in slightly different directions — dust being left behind in the comet's orbit (so that it often forms a curved tail) and the ion tail (gas) always pointing directly away from the Sun, since the gas is more strongly affected by the solar wind than dust is, and follows magnetic field lines rather than an orbital trajectory. While the solid body of the comet (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the tails can extend over 150 million km or more.
Both 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 due to 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.
Comets have highly elliptical orbits. Note the two distinct tails.
Surprisingly, cometary nuclei are among the blackest 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  (http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html). 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  (http://www.kvi.nl/~bodewits).
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. Single-apparition comets have parabolic and hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun. 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. 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 the most massive planet by far.
Also because of gravitational interactions, a number of periodic comets discovered in earlier decades or previous centuries are now lost, since their orbits were never known well enough to know where to look for their 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, which was discovered in 1869 but became unobservable after 1908 due to perturbations by Jupiter, and was not found again until accidentally rediscovered by LINEAR in 2001.
History of comet study
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  (http://classics.mit.edu/Aristotle/meteorology.1.i.html), 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. 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.
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. 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.
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.
The first suggestion that Kepler's laws of planetary notion should also apply to the comets was made by William Lower in 1610. In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, and Jean-Dominique 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.
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 his Principia Mathematica of 1687, Isaac Newton 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.
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. (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. 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. 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. By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of January 2005, 164 comets have achieved this distinction, though several have since been destroyed or lost.
Studies of physical characteristics
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. 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.
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 ocurred 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. 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. 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.
Forthcoming space missions will add greater detail to our understanding of what comets are made of. The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. In 2005, the Deep Impact probe will blast a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta space probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
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.
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.
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. The Comet Halley is expected to return in the year 2061. As yet, the 21st century has not seen the arrival of any great 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 an unstable orbit entirely between Jupiter and Saturn. 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3. 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  (http://comets.amsmeteors.org/meteors/showers/andromedids.html).
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 Sun-grazing comets, 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. However, it has been suggested that the object responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Comets in fiction
Comets are popular subjects for science fiction authors and filmmakers although they are often misrepresented as fiery objects, rather than icy.