Comets are small astronomical bodies that are composed chiefly of dust and ice crystals that orbit the Sun along highly elliptical orbital paths. As comets approach the Sun a portion of the comet is vaporized to form a characteristic head and tail.

Observations of comets date back to the fifth century B.C., when the Greeks mistook comets for clusters of stars. Aristotle (384-322 B.C.) argued that comets arose from the Earth. During the 18th century French mathematician Joseph-Louis Lagrange (1736-1813) postulated that comets were expelled from planets. Significant theories of comets include the hypothesis of British astronomer R. A. Lyttleton in 1948, that a comet was actually a "flying gravel-bank" of dust particles. In 1950, American astronomer Fred L. Whipple countered with the "dirty iceball" theory. Whipple believed the nucleus of a comet was a conglomerate of rocky fragments and "ices" made from frozen methane, ammonia, carbon dioxide...