Comets are comprised of icy mixtures of water, other chemicals, dust, and rocky materials. Asteroids are also made of rocky materials but they also contain metals. Because both celestial bodies orbit our sun, the water and ice in a comet can melt when it gets close to the sun. But asteroids, since they are mostly rock and metal, remain mostly unchanged.
Scientists believe both comets and asteroids are “leftovers” from the formation of our Solar System approximately 4.5 billion years ago and that they may have more things in common than they have differences.