comet

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Small, icy body orbiting the Sun, usually on a highly elliptical path that takes it beyond the dwarf planet Pluto. A comet consists of a central nucleus a few kilometres across and is made mostly of ice mixed with gas and dust. As a comet approaches the Sun its nucleus heats up, releasing gas and dust, which form a coma (comet head) up to 100,000 km/60,000 mi wide. Gas and dust stream away from the coma to form one or more tails, which may extend for millions of kilometres. Some comets, such as Halley's comet, stay within Pluto's orbit for most of the time.

Comets are of many different types, characterized by their orbits, their composition (the ratio of ice to dust, and the amount of frozen volatiles other than water ice, such as methane and carbon monoxide), and their size. Most comets approach the Sun on a hyperbolic orbit and are seen only once; others (the periodic comets) return regularly in elliptical orbits. Famous examples of the periodic comets are Halley's comet, which has a period of 76 years and is one of the largest comets known, with a nucleus about 15 × 7 × 7 km/9 × 4 × 4 mi across, and comet Encke, which has one of the shortest periods, at only 3.3 years.

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