Comets are small celestial bodies, consisting mainly of dust and gases that move in an elongated elliptical or nearly parabolic orbit around the sun. When a comet is far from the sun, it consists of a dense solid body or conglomerate of bodies called the nucleus, made up of frozen gases with particles of heavier substances interspersed throughout. This is a few miles in diameter. As it approaches the sun, a luminous "cloud" of dust …showed first 75 words of 634 total…

…showed last 75 words of 634 total…by composition and reflectivity, most being one of three types. The majority (C-type) are similar to carbon-chrondite meteorites with approximately the same composition as the sun (excluding hydrogen) and are relatively dark. Those with a composition of nickel iron mixed with silicates of iron and magnesium (S-type) are relatively bright. The M-type asteroids are composed of nickel iron and are bright.(Seeds, 401, 405) Work Cited: Seeds, Michael A.. Horizons: Exploring the Universe. 7th ed. Brooks/Cole, 2002