Asteroids vary greatly in size, from small boulders to hundreds of miles in diameter. The asteroid belt contains more than 200 asteroids larger than 60 miles wide. Scientists estimate that there are more than 750,000 asteroids in the belt wider than 3/5 mile. There are millions of smaller asteroids. The average distance between asteroids in the main belt is about 600,000 miles.
Ceres, the largest and most massive, is 590 miles wide. It contains almost one-third of the total mass of all the asteroids. At the opposite extreme, one of the smallest asteroids, discovered in 1991 and named 1991 BA, is only about 20 feet across.
Most asteroids follow elliptical (oval-shaped) orbits in the main asteroid belt, but many orbit outside the belt. For example, a number of asteroids called Trojans follow the same orbit as Jupiter, with one group ahead of the planet and one group behind. Three groups of asteroids – Atens, Amors, and Apollos – orbit in the inner solar system and are known as near-Earth asteroids. The orbits of some near-Earth asteroids cross the path of Mars, while others cross Earth's orbit. The Centaurs are another family of asteroids which orbit the Sun between Jupiter and Neptune.
Studies of reflected light from asteroids and analyses of meteorites found on Earth have provided information about their composition. Astronomers classify asteroids into three main groups:
• Carbon-rich asteroids dominate the outer part of the main belt. These C-types make up 75% of known asteroids. Their composition has not changed much since the solar system formed.
- • S-type or stony asteroids are more common toward the inner region of the belt. The have changed quite a lot over time due to melting and reformation. They make up about 17% of the asteroid population.
- • Metal-rich or M-type asteroids are thought to be pieces of the metallic core of bodies that were fragmented by impacts. Many are made of nickel-iron and are believed to be the source of iron meteorites.