There is another theory that the asteroids are fragments left by the disintegration of a planet that once existed between Mars and Jupiter. However, it is more likely that they are debris left over from the time of the Solar System's formation.
Most asteroids are in the region between the orbit of Mars and the orbit of Jupiter.
It is possible that the Earth will some day collide with an asteroid - it has happened in the past. No one knows how many asteroids there are, but it is thought that they number many thousands. About 10,000 are known at present. Large asteroids now rarely come within a million kilometres of each other, but collisions between asteroids must have occurred in the past, to produce the scattered fragments known as meteoroids. Many scientists believe that it was an asteroid that caused the extinction of the dinosaurs around 64 million years ago.