The word comet came to English by way of the Latin word cometes. This word, in turn, came from the Greek word κόμη, which means "hair of the head". The Greek scientist and philosopher Aristotle first used the derived form of κόμη, κομήτης, to describe what he saw as "stars with hair." The astronomical symbol for comets is (☄), consisting of a small disc with three hairlike extensions.
- Date: before 12th century
- 1 : a celestial body that appears as a fuzzy head usually surrounding a bright nucleus, that has a usually highly eccentric orbit, that consists primarily of ice and dust, and that often develops one or more long tails when near the sun
A comet is an icy small Solar System body that, 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 are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across. Comets have been observed since ancient times and have historically been considered bad omens.
Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands 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 cloud of icy bodies in the outer Solar System that were left behind during the condensation of the solar nebula. 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. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories.
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. These have somewhat blurred the distinction between asteroids and comets (see centaurs and asteroid terminology).
As of May 2010 there are a reported 3,976 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".