What is the difference between an asteroid and a comet? The main difference between asteroids and comets is what they are made of. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago. Asteroids formed much closer to the sun, where it was too warm for ices to remain solid. Comets formed farther from the sun where ices would not melt. Comets which approach the sun loose material with each orbit because some of their ice melts and vaporizes to form a tail.