About 3,000 asteroids have been cataloged. Asteroids range in size from tiny pebbles to about 578 miles (930 kilometers) in diameter (Ceres). There are about 40,000 known asteroids that are over about 0.5 miles (1 km) in diameter in the asteroid belt. Sixteen of the 3,000 known asteroids are over 150 miles (240 km) in diameter. There are many smaller asteroids. Some large asteroids even have orbiting moons. None of the asteroids have atmospheres.

The asteroid belt is a doughnut-shaped concentration of asteroids that orbit the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370 million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroid belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years. The strong gravitational force of the planet Jupiter shepherds (guides) the asteroid belt, pulling the asteroids away from the Sun, keeping them from falling into the inner planets.

The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are orbits where the gravitational forces from Jupiter do not let asteroids orbit (if there were asteroids there, they would be pulled towards Jupiter). The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866.

The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of the Earth's Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet.

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