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.
