Comet

Article By:

Brandt, John C. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado.

Last reviewed:February 2019

DOI:https://doi.org/10.1036/1097-8542.151000

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• Comet, published January 2014:Download PDF Get Adobe Acrobat Reader

A common kind of celestial object made of rock and ice that typically moves in a closed orbit around a star, such as the Sun, and appears as a hazy streak when viewed in Earth’s skies. Physically, a comet is a small, solid body, termed the nucleus, which comes in a range of sizes but is typically of the order of 3 km (2 mi) in diameter. Although it is mostly rocky, a comet’s nucleus contains a high fraction of icy substances, leading to popular descriptions of comets as “dirty snowballs” or “icy dirtballs.” Comets exhibit their iconic morphology of a bright ball, called the coma, followed by a streaking tail or tails (Fig. 1) when these ices heat up and sublimate during cometary approaches to the Sun. Compared to the orbits of planets and asteroids, comets’ orbits are more eccentric and have a much greater range of inclinations to the ecliptic (the plane of the Earth’s orbit). About 10 reasonably bright comets are discovered or rediscovered each year. On average, one comet per year is bright enough to be visible to the unaided eye and to generate interest among the public as well as the astronomical community. See also: Asteroid; Astronomy; Earth; Ecliptic; Heat; Planet; Sublimation; Sun

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