An asteroid is a lump of rock that circles the Sun. Asteroids are also called minor planets. The biggest one, Ceres, is only 580 miles (930 km) across.
There are three main kinds of asteroids, made up of rock, or metal, or a mixture of the two. The rocky ones, known as carbonaceous (C-types), are usually dark and difficult to spot. The lighter-colored ones, known as silicaceous (S-types), contain some metal. The pure metal M-types are the brightest and rarest.
Most asteroids circle the Sun in a broad band between Mars and Jupiter called the asteroid belt, which is about 215 million miles (345 million km) wide. The asteroids are pieces left over from when the major planets formed from small chunks of rock. Some of the asteroids travel outside the belt, moving out toward Saturn or in toward Earth. Asteroids that come close to Earth are called Near-Earth Objects (NEOs).