Asteroids are small pieces of rock that move around the sun between Mars and Jupiter. Over 200 asteroids have a diameter of more than 100 km and thousands of smaller ones exist. The largest asteroid that man knows of is Ceres, with a diameter of 1,100 km.

It is not clear how asteroids developed. Scientists today think that they may be in a place where a planet should have formed. But such a planet couldn't form because of the power of Jupiter. Another theory says that the asteroids were a big planet that was destroyed in a collision a long time ago.

There are also other asteroid belts in the solar system. New asteroids have been found between Saturn and Uranus. Over 400 asteroids have been found near the Earth and Venus. NASA sends probes to asteroids when they are close to Earth, in order to discover what they are made of.

Asteroids that are on a course that may hit the Earth are called meteorites. Most of them burn up in our atmosphere but some of them are big enough to hit the Earth. About 65 million years ago, a large meteorite crashed into the Earth. Scientists believe that a lot of dust and rock was stirred up and made our planet very dark - so sunlight couldn't get in. It became colder and colder and the dinosaurs probably died out this way.

Vesta - one of the largest asteroids
Image: NASA / JPL / MPS / DLR / IDA / Björn Jónsson,
Public domain, via Wikimedia Commons