A comet is a relatively small astronomical object similar to an asteroid but composed largely of ice. In Earth's solar system, the orbits of comets may extend past that of Pluto; of the comets which enter the inner solar system, most have relatively highly elliptical orbits. Often described as "dirty snowballs", comets are composed largely of frozen carbon dioxide, methane and water with dust and various mineral aggregates mixed in.
Both coma and tail are spectacularly illuminated by the Sun and are sometimes 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. 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.
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years. 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 father 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 WayGalaxy; and the Sun's hypothetical companion star Nemesis, an unknown Planet X.
Ironically, 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 spectacularly bright outgassing.
In 1996, comets were found to emit X-rays. These X-rays surprised researchers, because X-rays are typically emitted by hot objects and comets are cold. The X-rays were 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.
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 called 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 example) while one has been observed to have collided with another object (Comet Shoemaker-Levy 9 which hit Jupiter in 1994). Some comets orbit in groups and are thought to be pieces of a single object such as the Sun-grazing comets.
It was not settled whether comets are atmospheric phenomena or interplanetary objects until the 16th century, when measurements of parallax taken by Tycho Brahe revealed that they must be moving outside of the Earth's atmosphere. In the 17th century, Edmond Halley used the theory of gravitation, recently developed by Isaac Newton, to try to calculate the orbits of comets. He then found that one of them periodically returned to the vicinity of the sun every 76-77th year. Soon this comet became known as Comet Halley, and from ancient sources it is known to have been observed by humans since at least 66 BC.
The second comet to be discovered to have a periodic orbit was Comet Encke, in 1821. Like Halley's comet, it is named after the calculator, the German mathematician and physicist Johann Franz Encke, that found it to be a periodic comet. Usually, comets get their names after their discoverer(s). Encke's comet has the shortest period of any comet, only 3.3 years, and because of this also the largest number of recorded appearances. It was also the first comet whose orbit was noticed to be influenced by non-gravitational forces (see below). Although it is now usually too faint to be visible with the naked eye, it may have been a bright comet a few thousand years ago, before most of its surface ice had evaporated. So far, it is not known to have been observed before 1786, but improved analysis of its earlier orbit might show that it corresponds to observations mentioned in ancient sources. If it is a very old comet, its only naked-eye visibility would however have been many thousands of years ago. The time would be even longer if it was originally a large comet, such as Comet Hale-Bopp, and if the comet was even larger, for instance a Centaur, it would take hundreds of thousands of years to wear out.
The actual nature of comets was speculated over for centuries. In the early 19th century another German mathematician, Friedrich Wilhelm Bessel, was on the right track. He created a theory about the brightness of the comets coming from the evaporation from a solid object and that the non-gravitational forces of Comet Encke were caused by the jet forces created as the material evaporated from the surface of the object. His idea was forgotten for more than 100 years, before Fred Lawrence Whipple independently proposed the same idea in 1950. It soon became the accepted comet model and 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 212001 and confirmed that the characteristics of Comet Halley are common in other comets as well.
Because of non-gravitational forces and also the gravitation influence of a massive planet like Jupiter, it is easy for a periodic comet's orbit to be perturbed. For this reason, a number of periodic comets discovered in earlier decades or a previous century 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.
The Stardust spacecraft was launched in February 1999 and collected particles from the coma of Comet Wild 2 in January 2004. Sample material will be returned to Earth by a capsule in 2006.