Comets are basically “snowy dirtballs” or “dirty snowballs”—clumpy collections of rocky material, dust, and frozen water, methane, and ammonia that move through the solar system in long, highly elliptical orbits around the Sun. When they are far away from the Sun, comets are simple, solid bodies; but when they get closer to the Sun, they warm up, causing the ice in the comets’ outer surface to vaporize. This creates a cloudy “coma” that forms around the solid part of the comet, called the “nucleus.” The loosened comet vapor forms long “tails” that can grow to millions of miles in length.
Comet C/2002 V1 (also known as NEAT) as seen in a Kitt Peak Observatory photograph. (NASA)