What is an asteroid?

Billions of asteroids orbit the Sun. Over 90 percent make up the Asteroid Belt, a doughnut-shaped ring between Mars and Jupiter. They are sometimes called the minor planets, as they are each rocky bodies orbiting the Sun. They are the leftovers of a planet that did not form when the Solar System was young, 4.6 billion years ago.

The first to be discovered, Ceres in 1801, is also the largest. It is spherical and about 932 km wide. Most asteroids are much smaller, about one billion are more than one kilometer wide, but many more are only meters across. Asteroids are made of rock, metal or a combination of the two.

Over ten thousand individual asteroids have been discovered and had their orbits recorded. Each has been given a name. One asteroid, called Ida, has a tiny moon of its own; this is the smallest known satellite in the Solar System. Our only close-up views have been provided by two space probes, the first to be seen was Gaspra, by the Galileo probe in October 1991. The NEAR (Near Earth Asteroid Rendezvous) probe showed us Mathilde in 1997 and Eros in 1998.