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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