Asteroids are chunks of rock, ranging in size from hundreds of miles down to a few metres across, that orbit the Sun like miniature planets. While a few of the largest asteroids have a roughly spherical shape, most of them are much more irregular in appearance. To date, more than a million asteroids have been discovered – the majority of them in the main asteroid belt sitting between the orbits of Mars and Jupiter. The asteroid belt is one of the Solar System’s most important features, marking the boundary between the two different types of planets: the rocky inner planets Mercury, Venus, Earth and Mars, and the giant outer planets Jupiter, Saturn, Uranus and Neptune. It’s also the widest swathe of the Solar System between the orbits of Mercury and Neptune that is entirely planet-free.
The lack of planets in the region between Mars and Jupiter was a puzzle for early astronomers, and many of them were convinced there had to be a hitherto unknown planet there. By the end of the 18th century, the hunt for this supposedly missing planet had become something of a competition between rival groups of astronomers. The apparent winner was Giuseppe Piazzi of Palermo Observatory in Sicily, who found a planet-like object in the right sort of area on 1 January 1801. But
