Ceres is now known to have a width of about 940 km - about one quarter (one fourth) the size of Earth's Moon. But it accounts for more than one quarter of the total weight of all the asteroids.
Another sixteen asteroids are more than 240 km across. However, most of the asteroids are rocks and boulders only a few metres across. If all of them were squashed into one object, they would still be smaller than the Moon. Some asteroids are stony, some are made of metals, such as iron and nickel. Most are black-brown in colour and rich in carbon.
Many asteroids belong to families. Each family has the same colour and seems to be made of the same materials. They also travel close together in different parts of the main belt. One of the largest of these is the Flora family, which has about 400 known members.
Asteroids are also found outside the main belt. They have been thrown out of their home by the strong gravity of Jupiter or by collisions with other asteroids. Some have become moons of the giant planet. Hundreds of thousands of asteroids – some large – fly close to Earth.