Comets are made up of some of the original material from which the solar system formed. Orbiting far from the Sun, this primordial material has survived in an unaltered state for billions of years. When a comet nucleus is gravitationally drawn into the inner solar system it begins to heat up. The volatile materials from which it is made boil off to form the head and tail(s) that have amazed, baffled, and frightened people throughout history. This tremendous light show is produced from just the small solid nucleus measuring only 15 or 20 kilometers long. Think of it as a very dirty iceberg! This view was confirmed by the spacecraft flybys of Halley's Comet by Japanese, Soviet and European spacecraft in 1986.
