NASA - National Aeronautics and Space Administration

Asteroids are small, rocky objects that orbit, or travel, around the sun. Most asteroids are found between the planets Mars and Jupiter. This area is called the asteroid belt. Astronomers, or scientists who study outer space, think there are about 30,000 asteroids in the asteroid belt.

Image to right: The asteroid Ida is about 35 miles (55 kilometers) long. It is one of thousands of asteroids in the asteroid belt, a region between the orbits of Mars and Jupiter. Credit: NASA

Asteroids come in many sizes. The largest asteroid, Ceres, is about 600 miles (970 kilometers) across. Ceres, the first known asteroid, was discovered in 1801. One of the smallest asteroids, 1991 BA, was discovered in 1991. It is only about 20 feet (6 meters) across. Most asteroids are less than 18 miles (30 kilometers) across.

Astronomers don't know exactly how asteroids were formed. They offer two possible explanations. Some think there was once a planet between the orbits of Mars and Jupiter. That planet broke up and formed the asteroids. Others think there were enough rocks and other materials between Mars and Jupiter to form a planet. But instead of forming a planet, the material became a belt of rocky objects -- the asteroid belt.

Astronomers divide asteroids into two groups. One group is made mostly of carbon. Carbon is a soft, black substance that is plentiful on Earth. Asteroids in the second group are rich in minerals. These asteroids formed from minerals heated by the sun.

How to cite this article: To cite this article, World Book recommends the following format: "Asteroid." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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