Comets

Comets are small rock, dust and ice objects that orbit the Sun. In 1950, Fred Whipple proposed the "dirty snowball" picture of comet structure. The nucleus of the comet - typically about 10 kilometers across - is composed mostly of dust and ices of carbon dioxide, water, ammonia and methane.

As the comet approaches the Sun, the ices in the nucleus sublime to form the coma, a dense cloud of gas and dust particles around the nucleus. The coma contains water, carbon dioxide and other neutral gases.

When comets are near the Sun, the pressure of the Sun's radiation on the coma blows the dust and gas in the coma back into a long tail or tails. Eventually, after the comet has made many orbits of the Sun, the coma will have been e

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