Asteroids were formed billions of years ago. Scientists think that asteroids may have started out as small planets. Or, they may have been pieces of what tried to become a single large planet, but were kept from coming together because of Jupiter's strong gravity. The asteroids do not travel in set orbits like the planets. They are pulled by the gravity of Jupiter and Mars.
Asteroids keep bumping in to each other, and when they do, pieces of rock break off.
These pieces may streak through the sky towards Earth. They look like bright fireballs and are now called meteors. When a large meteor hits the Earth, it makes a deep crater and then shatters into smaller pieces. Now that the pieces have hit the Earth, they have become what are called meteorites. Near the city of Winslow, Arizona, you can visit the Barringer Crater.