The Asteroid Belt, located between Mars and Jupiter, about 2-4 AU from the sun, separates the terrestrial planets from the outer planets. It holds most of the asteroids in our solar system. Several hundred asteroids have been discovered, but the total mass of all the asteroids is less than half the mass of the moon. Scientists now believe that the asteroid belt is not a broken-up planet but a place where a planet could have formed but was prevented of doing so by the gravitational interference from Jupiter.
The largest known asteroid, 1 Ceres, is 933 kilometers in diameter (580 miles). It makes up about 25 percent of the total mass of the asteroids. The next largest, 2 Palla, 4 Vesta and 10 Hygeia are between 400 and 525 kilometers in diameter (around 340 miles). All other known asteroids are less than 340 kilometers across. Astronomers believe that we know 99 percent of asteroids larger than 100 kilometers, but we only know about half of the asteroids from 10 to 100 kilometers, and only a very few of the asteroids smaller than that.
In the Asteroid Belt, there are relativley empty regions known as Kirkwood gaps. Kirkwood gaps are regions where an orbital period would be a simple fraction of that of Jupiter's. An object in that orbit would probably be thrown into another orbit by Jupiter.
Asteroids are classified in two ways. One is their chemical composition. There are three main chemical compositions: the C-type, the S-type, and the M-type. The C-type asteroids are usually very dark. Made up mostly of carbon, these make up about 75 percent of all the asteroids. The S-type asteroids are relativley bright, and are made up of nickel-iron, and iron- and magnesium-silicates. About 17 percent of the asteroids are S-type. The M-type asteroids are very bright, and are made up of pure nickel-iron. In addition to these three types, there are also a dozen or so other rare types.
The other way Asteroids are classified is by their location. There are also three main locations: the Main Belt, Near-Earth Asteroids (NEAs), and Trojans. The Main Belt, more widely known as the Asteroid belt, is divided into several sub-groups. Near-Earth Asteroids are divided into three sub-groups: Atens, Apollos, and Amors. Trojans orbit in Jupiter's lagrange points; several hundred are known, but scientists believe that there may be a thousand or more.
There are also a few asteroids in the outer solar system. These stray asteroids are known as Centaurs. Only a few have been found, but there are probably many more that haven't been discovered. These asteroids orbit's are unstable and are likely to be peturbed in the future, as they often cross other planet's orbits.
Only a few asteroids have been studied closely. Ida and 951 Gaspera were photographed by the Galileo spacecraft. The NEAR mission flew by 253 Mathilde, and orbited and eventually landed on 433 Eros in early 2000.
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