Comets are that look like dirty snowballs flying through space. They have tails of dust that may be over 6 miles long. Besides the tail, a comet has a nucleus, or center, made up of a closely packed ball of ice and . Surrounding the nucleus is a cloud of water and gases referred to as the coma. People can see comets only when they pass close to the sun. As they get closer to the , some of the ice melts and forms the long tail. Some comets after regular periods of time. Halley's Comet passes through the system every 76 years. It was last seen in 1986 and will appear again in 2062. Earth is in no from comets. When the passes through the comet's tail, small pieces of rock called fall into the atmosphere. Most of these are burned up in the mesosphere. They appear during a meteor shower as stars.