A comet is a small, icy body that orbits the Sun. It is made up of dust and frozen gases, such as water, ammonia, and methane. When a comet gets close to the Sun, its icy surface begins to melt and the gas and dust form a visible atmosphere called a coma. As the comet moves away from the Sun, the coma and tail dissipate. Comets often develop tails of dust, gas, and sometimes even ice particles that can be millions of kilometers long.