Asteroid Ida © NASA

Asteroids are large rocks orbiting the Sun that are too small to be called planets.

Many tens of thousands of asteroids have been discovered so far, and more than 12,000 of these have been officially named. The largest, Pallas, is 570 km in diameter. The smallest known asteroids are about 10km in diameter, but there are probably millions more that are smaller than this.

Most asteroids are found in the Asteroid Belt, between Mars and Jupiter. Some others have been seen to pass very close to Earth. These are called Near-Earth Objects or NEOs.

If you took all the asteroids and stuck them together, you would still have an object that is smaller than the Moon.