What are asteroids?
Tens of thousands of rocks, ranging in size from 1.6 km (1 mile) to 1000 km (600 miles) across, are flying about the Solar System. They are called asteroids, and are mostly found in a belt which lies far beyond the Earth, between the orbits of Mars and Jupiter. The Earth (along with the three other inner planets of the Solar System, Mercury, Venus and Mars) is struck by an asteroid every 200 000 years, or so scientists believe. Our Moon, which does not have an atmosphere to protect it from flying objects, is marked with thousands of impact craters.

Space rock
On its way to Jupiter, the space probe Galileo photographed a stony asteroid called Ida, which is 58 km (36 miles) long and 21 km (13 miles) wide.