Asteroids are smaller than any of the planets and measuring up to 900 km (560 miles) across. Asteroids are small rocky or icy bodies that orbit the Sun. Smaller rocks are called meteors. They are sometimes called minor planets. Most asteroids are found in an orbit between Mars and Jupiter, and more than 7,000 of them have been identified.
The term asteroid is usually applied to objects that are larger than 1.6 kilometers in diameter. One asteroid, called Ida, has a tiny moon all of its own. This is the smallest known satellite in the Solar System. Asteroids were probably formed at the same time as the planets.
Many asteroids have struck the Earth already, and many scientists believe that such an impact resulted in the extinction of the dinosaurs about 65 million years ago.