Asteroids are small, rocky bodies that orbit the sun. They are also called minor planets or planetoids. In general, they are materials left over from when the planets formed or from the collision and breakup of other bodies. Most travel around the sun in a loose path that falls between the orbits of the planets Mars and Jupiter. Astronomers identified the first asteroids in 1801. They have identified the orbits of more than 20,000 asteroids since then. In 1991, a satellite probe sent from Earth, Galileo, was the first to pass near and studied an asteroid.

Physical features

Asteroids are smaller than planets. Only about 30 have diameters of more than 124 miles (200 kilometers). The largest asteroid, Ceres, has a diameter of about 600 miles (965 kilometers), but millions of asteroids are the size of large boulders.

Asteroids can be almost any shape. Some are nearly round while others are irregularly shaped chunks. Asteroids are made up of stone and metal. About two thirds of asteroids contain large amounts of carbon. These asteroids appear dark because they reflect little light. The other third are brighter. They reflect light better because they include more iron and other metals.

Asteroids and the Earth

Many small asteroids enter the Earth's atmosphere each day. Almost all burn up because of the friction with gas molecules in the air. An asteroid that survives the fall through the atmosphere and reaches the Earth's surface is called a meteorite. During a million-year period, the Earth will collide with only a few asteroids with diameters of more than 0.6 mile (1 kilometer).

A collision with a large asteroid could have serious consequences. Many astronomers think that an asteroid 6 to 12 miles (10 to 20 kilometers) in diameter struck the Earth about 65 million years ago in what is now southern Mexico. It may have caused such a tremendous dust cloud that it blocked sunlight from reaching the Earth for several years. All over the world, plants would have died. This may explain the extinction of many animals that relied on the plants for food, including the dinosaurs.