The current theory on the composition of comets came from Fred Whipple in 1950. It is called the dirty snowball theory. At the center of the comet's head is the nucleus, typically several kilometers in diameter. This nucleus is composed of ices of water, ammonia, carbon dioxide and methane with dust mixed in. The dust particles are smaller than the particles in cigarette smoke.

     Many comets have two tails, a gas tail and a dust tail and they may extend 150 million km. Both always point away from the Sun. The dust tail is caused by dust particles released from the gasses being vaporized. The dust tail usually curves slightly behind the comet. The gas, or ion tail is composed of ionized gasses excited by the ultraviolet radiation from the Sun. This tail is a wispy bluish color and is blown straight back. The gas tail is brighter than the dust tail.