Comet

   The nucleus of a comet is only a few miles across, and made of mostly water-ice, with some methane and ammonia. As the comet leaves the area of the Oort Cloud and falls into the orbit of the outer planets, the volatile elements of the nucleus boil. This creates a cloud-like head called a coma, which can be tens of thousands of kilometers across. Solar wind from the Sun pushes the coma back, creating the comet's tail. The tail always points away from the Sun. In most cultures, the comet represents fear, and foretells death or some great catastrophe. See Comet Shoemaker-Levy 9, Cometophobia, Halley, Edmond, Kuiper Belt, Long Period Comets, Meteoroid, Moon, The, Oort Cloud, Period, Short Period Comets and Tunguska.