A comet is an icy, small Solar System body which, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. Comets have been observed since ancient times and have traditionally been considered bad omens.
Comets are distinguished from asteroids by the presence of a coma or a tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.
As of January 2011 there are a reported 4,185 known comets of which about 1,500 are Kreutz Sungrazers and about 484 are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular. Particularly bright or notable examples are called "Great Comets".
Comet nuclei are known to range from about 100 meters to more than 40 kilometres across. They are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia. Because of their low mass, comet nuclei do not become spherical under their own gravity, and thus have irregular shapes. Officially, according to NASA guidelines, a comet has to be at least 85% ice in order to be considered an actual comet.
Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. 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 to the naked eye.
As a result of outgassing, comets leave a trail of solid debris. If the comet's path crosses Earth's path, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October.
Most comets have elongated elliptical orbits that take them close to the Sun for a part of their orbit, and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: the longer the period the more elongated the ellipse. Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years.
Short-period comets are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. At the shorter extreme, Encke's Comet has an orbit which never puts it farther away from the Sun than Jupiter. Short-period comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years). Short-period comets originate in the Kuiper belt, or its associated scattered disc, which lie beyond the orbit of Neptune.
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the solar system. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods". Long-period comets plunge towards the Sun from the Oort Cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars.
Single-apparition comets are similar to long-period comets since they also have parabolic or lightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change, and when they are beyond the planets, their osculating eccentricity is still hyperbolic with aphelia lying beyond the outer Oort Cloud. The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)). All comets with parabolic and slightly hyperbolic orbits belong to the Solar System and had certain orbital periods, generally hundreds of thousand, or millions of years before being perturbed onto an ejection trajectory. Only a few hundred comets have been seen to achieve a hyperbolic orbit when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System. No comets with an eccentricity significantly greater than one have been observed, so there are no confirmed observations of comets that are likely to have originated outside the Solar System.
Some authorities use the term periodic comet to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), while others use it to mean exclusively short-period comets. Similarly, although the literal meaning of non-periodic comet is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).
Some comets meet a more spectacular end—either falling into the Sun, or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Earth's Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.
Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth (about 4 billion years ago) brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Other researchers have cast doubt on this theory. The detection of organic molecules in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. There are still many near-Earth comets, although a collision with an asteroid is more likely than with a comet.
It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.
Forthcoming space missions will add greater detail to our understanding of what comets are made of. The European Rosetta probe is presently en route to Comet Churyumov–Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.
Of the thousands of known comets, some are very unusual. Encke's Comet orbits from outside the main asteroid belt to just inside the orbit of the planet Mercury while the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is 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. Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. Comet C/2010 X1 (Elenin) is a long-period comet discovered by amateur Russian astronomer Leonid Elenin on December 10, 2010, remotely, using the International Scientific Optical Network's robotic observatory near Mayhill, New Mexico, U.S.A. At the time of discovery Elenin had an apparent magnitude of 19.5, making it about 150,000 times fainter than the naked eye magnitude of 6.5. The discoverer, Leonid Elenin, estimates that the comet nucleus is 3–4 km in diameter.
However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO's 2000th comet was discovered by Polish amateur astronomer Michal Kusiak on 26 December 2010, and the numbers are expected to continue rising steadily for the foreseeable future.