Any of a class of small icy objects orbiting the Sun and developing diffuse gaseous envelopes and often long glowing tails when near the Sun. They are distinguished from other objects in the solar system by their composition, hazy appearance, and elongated orbits. Most comets originate in the Oort cloud or in the Kuiper belt. Other bodies' gravity can alter their orbits, causing them to pass close to the Sun. Short-period comets return in 200 years or less, others in thousands of years or not at all. A comet typically consists of a small, irregular nucleus, often described as a “dirty snowball,” with dust and other materials frozen in water mixed with volatile compounds. When one nears the Sun, the heat vaporizes its surface, releasing gases and dust particles, which form a cloud (coma) around the nucleus. Material in the coma may be pushed away from the Sun by its radiation and the solar wind, forming one or more tails. Meteor showers occur when Earth passes through dust left by the passage of a comet.

This entry comes from Encyclopædia Britannica Concise.
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