A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger, and if sufficiently bright may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many different cultures.

Comets have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space along hyperbolic trajectories.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). 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.[1] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[2][3] The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.

As of July 2013[update] there were 4,894 known comets,[4] and this number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System may number one trillion.[5] Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular.[6] Particularly bright examples are called "Great Comets".

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek ("wearing long hair"), and the Oxford English Dictionary notes that the term () already meant "long-haired star, comet" in Greek. was derived from ("to wear the hair long"), which was itself derived from ("the hair of the head") and was used to mean "the tail of a comet".[8][9]

The astronomical symbol for comets is (), consisting of a small disc with three hairlike extensions.[10]

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[11] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[12] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[13]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[14][15] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[16] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[17][18]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[19] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0% of the light that falls on it;[19] by comparison, asphalt reflects seven percent of the light that falls on it. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes.[20]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[27] but ascertaining their exact size is difficult.[28] The nucleus of P/2007 R5 is probably only 100200 metres in diameter.[29] A lack of smaller comets being detected despite the increased sensitivity of instruments has lead some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[30] Known comets have been estimated to have an average density of 0.6 g/cm3.[25] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[31]

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Comet - Wikipedia, the free encyclopedia