Asteroids are actually very tiny planets which revolve in orbit around the sun. Thousands have been seen by astronomers and many have been named. But new asteroids are being discovered almost daily.
Sometimes, because of the attraction of other planets, these asteroids change their orbit and collide with other asteroids. The fragments that break off as a result of this collision and fall to earth without being burned up are called meteorites.
Meteorites hit the earth anywhere from every day to once in 10,000 years, depending on their size. Specks of stardust, for example, are tiny meteorites which settle on our earth every day. Larger ones, about the size of a boulder, fall to earth about 1,500 times every year. But the really big meteorites, weighing upwards of 50,000 tons, are believed to hit only once in every 10,000 years.
A gouged-out crater in Africa, 26 miles across, was made over a quarter billion years ago by the largest meteorite (about one mile wide) ever to hit the earth.