There are many rocky fragments in our Solar System that have never been swept together to form planets. Most of the larger ones - called asteroids - swarm in a belt between Mars and Jupiter, although a few have paths that take them near to the Earth; these are known as Apollo asteroids.

The largest asteroid, called Ceres, is over 1,000 kilometres across. It is thought that there may be half a million asteroids with diameters bigger than 1 kilometre and countless numbers that are smaller.

Asteroids are most likely fragments from the earliest days of the Solar System. It is probable that many asteroids were once larger, but because they were travelling so close to each other, many must have collided and broken up into the fragments that we see today. Asteroids mostly seem to be made of the same rocky materials that can be found on Earth.

On average, a house-sized Apollo asteroid hits the Earth every century; the last one burst in the sky over Russia in 1908. An asteroid big enough to threaten life on Earth hits the planet only once every 50 million years or so. The last time this happened may have been the cause of dinosaurs becoming extinct on Earth.