A body, probably resembling a "dirty snowball" between 0.1 and 100 miles across that travels through the solar system in an elliptical orbit of random inclination. A comet grows a tail if it comes close enough to the sun.

These minor members of our solar system are thought to consist of lumps of dirt and ice which have existed since the formation of our solar system. The celestial objects number in the millions are believed to reside in the Oort Cloud, which is a spherical region beyond Pluto and is centered around our sun. As comets enter our solar system, the heat of the sun begins to vaporize the icy material and the resulting vapor and dust develops into a long glowing tail that can be hundreds of thousands of miles in length. Comet literally means 'long haired one' and is derived from the Greek kome which meant 'hair' as it was applied metaphorically to the tail of a comet that was seen as streaming out behind as though it was blowing in the wind.