A meteor forms when a meteoroid — which is a fragment of an asteroid — enters Earth's atmosphere after orbiting the sun. The atmosphere's hot air helps the meteor evaporate, and what ends up on the ground or in the water — called a meteorite — is much smaller than what initially entered the atmosphere, Heck said. About 60,000 meteorites, which can be as small as pebbles, have been discovered on Earth, and some are on display at the Field Museum.