A comet is an icy
small Solar System body that, when passing close to the
Sun, warms and begins to release gases, a process called
outgassing. This produces a visible atmosphere or
coma, and sometimes also a
tail. These phenomena are due to the effects of
solar radiation and the
solar wind acting 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 may be up to 15 times the Earth's diameter, while the tail may stretch one
astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.
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 part 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. 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
centaur minor planets has blurred the
distinction between asteroids and comets.
As of November 2014 there are 5,253 known comets, a number that is steadily increasing as they are discovered. 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 (in the
Oort cloud) is estimated to be one trillion. Roughly one comet per year is visible to the
naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "
great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet
Tempel 1 to study its interior.
The word comet derives from the
Old Englishcometa from the
Latincomēta or comētēs. That, in turn, is a
latinisation of the
Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant "long-haired star, comet" in Greek. Κομήτης was derived from κομᾶν ("to wear the hair long"), which was itself derived from κόμη ("the hair of the head") and was used to mean "the tail of a comet".