Earlier Later This site tells the story of the history of the universe. Click Earlier and Later to follow the story.
Comets are lumps of ice made of water , carbon dioxide , methane and ammonia .
Frozen in this ice is dust and small lumps of rock . Comets are probably similar to the planetesimals from which the planets formed. Therefore there is great interest in learning more about them. Unfortunately most of them are far away from Earth . There seem to be two sources of comets.