A comet is composed of dust, ammonia, ice, carbon dioxide and methane in addition to other substances. Ice and dust covered with an organic substance largely make up the nucleus of a comet. The tail of a comet consists of dust and gas.
The largest-known comet is a fraction of the size of Pluto, a dwarf planet. A comet is typically about the size of a little town, with a nucleus of only about 10 miles. The tail of a comet, however, can span up to 100 million miles. When a comet gets close to the sun, it heats up and forms an atmosphere known as a coma, which can be 1 million miles wide. Comets have neither moons nor rings.