Occasionally people wonder whether the belt was made up of the remains of a destroyed planet, or a world that didn't quite get started. However, according to NASA, the total mass of the belt is less than the moon, far too small to weigh in as a planet. Instead, the debris is shepherded by Jupiter, which kept it from coalescing onto other growing planets.
Observations of other planets are helping scientists to better understand the solar system. According to a developing theory known as Grand Tack, in the first 5 million years of the solar system, Jupiter and Saturn are thought to have moved inward toward the sun before changing direction and heading back to the outer solar system. Along the way, they would have scattered the original asteroid belt before them, then sent material flying back to refill it....
...Asteroids are classified into several types based on their chemical composition and their reflectivity, or albedo.
- C-type asteroids make up more than 75 percent of known asteroids. The "C" stands for carbon, and the surfaces of these extremely dark asteroids are almost coal-black. Carbonaceous chondrite meteorites on Earth have a similar composition, and are thought to be pieces smashed off of the larger asteroids. While C-type asteroids dominate in the belt, according to the European Space Agency, they make up only about 40 percent of the asteroids closer to the sun. These include subgroups of B-type, F-type, and G-types.
- S-type asteroids are the second most common type, making up about 17 percent of known asteroids. They dominate the inner asteroid belt, becoming rarer farther out. They are brighter and have metallic nickel-iron mixed with iron- and magnesium-silicates. The "S" stands for silicaceous.
- M-type asteroids ("M" for metallic) are the last major type. These asteroids are fairly bright and most of them are composed of pure nickel-iron. They tend to be found in the middle region of the asteroid belt.
- The remaining rare types of asteroids are A-type, D-type, E-type, P-type, Q-type, and R-type.
