Asteroids are often known as 'minor planets', since they are too irregular in size, too numerous and too small to be considered as proper planets. There are thought to be billions of these rocky objects orbiting the Sun.

Since they can only be seen as points of light in telescopes, British astronomer William Herschel coined the term 'asteroid' - from a Greek word meaning 'starlike' - to describe this new class of celestial objects. Earlier this century, astronomers coined a less poetic term for them - 'vermin of the skies' - as they regularly appeared in photographs of distant galaxies and nebulas, spoiling the sensitive observations.

The largest of the asteroids, Ceres, was the first to be discovered. It was found by Giuseppi Piazzi from Palermo, Sicily on 1 January 1801. A second minor planet, Pallas, was found by Wilhelm Olbers in 1802, followed by Juno (1804) and Vesta (1807). Nearly four decades passed without any further discoveries, but this changed with the introduction of improved star maps and better telescopes. From 1847 onwards, not a year has passed without the discovery of at least one asteroid.

Ceres is now known to have a diameter of about 940 km, only about one quarter the diameter of Earth's Moon. However, it contains more than one quarter of the total mass of all the asteroids. Another sixteen have diameters greater than 240 km. The smallest are like fragments of rock and boulders a few metres across. If all the asteroids were squashed into one object, they would still be smaller than the Moon.

Asteroid Moons