Most meteorites come from asteroids. Somewhere out in our solar system pieces of asteroid collide, putting them on a collision course with the Earth. When they reach the Earth's atmosphere most of them disintegrate into dust and their arrival is barely noticed. But sometimes they make it through the atmosphere and chunks of rock or iron land on our planet. We call these meteorites. Occasionally their arrival has major consequences. Very large ones can make big craters and cause massive changes to the Earth's climate. Fortunately this is very rare. Most are quite small land in the sea or in uninhabited areas and are never found by humans. This photo shows Meteor Crater in Arizona, USA, which is believed to have been created when a meteorite landed about 50,000 years ago.