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A celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. Comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water.

[Middle English comete, from Old English comēta, from Late Latin, from Latin comētēs, from Greek komētēs, long-haired (star), comet, from komē, hair.]

com′et·ar′y (-ĭ-tĕr′ē), co·met′ic (kə-mĕt′ĭk) adj.
ThesaurusAntonymsRelated WordsSynonymsLegend:
Adj.1.cometary - of or relating to or resembling a comet
References in classic literature ?
The public temper will soon get to a cometary heat, now the question of Reform has set in.
The outline which would bound my walks would be, not a circle, but a parabola, or rather like one of those cometary orbits which have been thought to be non-returning curves, in this case opening westward, in which my house occupies the place of the sun.
It's been half-way to Jupiter, executed picture-perfect flybys of an asteroid and a comet, collected cometary material for return to Earth, then headed back out into the void again, where we asked it to go head-to-head with a second comet nucleus," said Tim Larson, project manager for Stardust-NExT at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
Now a research team finds that all the captured cometary grains formed near the Sun and somehow got transported to the outer solar system, where they mingled with the prevailing ices to create Comet Wild 2.
This is the first time that astronomers have viewed a cometary breakup in such detail, notes Harold A.
If the activity in Scheila proves to be cometary in nature, this will be only the sixth known main-belt comet, and about 100 times larger than previously identified main belt comets," explained Larson.
They were one of three research teams who found the cometary material to have a geometric albedo of about six percent.
Furthermore, people have postulated--and it's just a hypothesis--that materials like this would have seeded the Earth from cometary tails that passed by the Earth early on after its formation," says Jewell.
The cometary debris ionised the atmospheric gas it passed through, boosting the current that normally flows between the ionosphere - an electrically charged region in the upper atmosphere - and the ground, according to him.
Deep Impact will be sent on its way toward its new cometary target on New Year's Eve, when it will swing past the Earth 29,000 km above Mongolia.
However, the ratio of deuterium to hydrogen in cometary water is much higher than in water on our planet.
As a wayward Centaur approaches the sun, its heat begins to evaporate the icy contents, resulting in a cometary tail.