Comets are small, fragile, irregularly shaped bodies composed mostly of a mixture of water ice (ice composed of H20), dust, and carbon- and silicon-based compounds. They have highly elliptical orbits that repeatedly bring them very close to the Sun and then swing them into space. Comets have three distinct parts: a nucleus, a coma and a tail. The solid core is called the nucleus, which develops a coma with one or more tails when a comet sweeps close to the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun. The coma and tails of a comet appear only when the comet is near the Sun.
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