How Asteroid Belts Work

Asteroid Characteristics
Asteroid 951 (Gaspra) as seen by the Galileo spacecraft in 1991.
Asteroid 951 (Gaspra) as seen by the Galileo spacecraft in 1991.
Image courtesy NASA

The majority of the asteroids in the main asteroid belt fall under three categories:

C-type (carbonaceous) - These make up about 75 percent of all known asteroids. C-type asteroids are actually thought to be similar in composition to the sun, just without hydrogen, helium and other combustible material. They're very dark and absorb light easily, and you can locate them on the outer edges of the main belt.

S-type (silicaceous) - These make up about 17 percent of all known asteroids. Their composition is mainly metallic iron and iron-magnesium silicates, and they're found in the inner edge of the main belt.

M-type (metallic) - The remaining 8 percent of the asteroids are made of metallic iron and are found in the middle region of the main belt.

Asteroids typically travel in a slightly elliptical orbit around the sun in the same direction as the Earth. They rotate simply, much like the Earth, except over a much short period of time -- anywhere between one hour and a day, depending on their size. Interestingly, most asteroids larger than 200 meters spin very slowly, no faster than once every 2.2 hours. This led astronomers to assume that bigger asteroids are very loosely held together because of constant bombardment from other asteroids. If they spin any faster, they'll break apart and fly out into space. It's suggested that asteroid 253 (Mathilde) is about as dense as water, even though it's 52 kilometers wide.

Many people might be surprised to learn that most asteroids in the main belt are only the size of a pebble. Despite the vast amount of space it takes up, astronomers estimate the total mass of the entire main asteroid belt to be less than 1/1,000th the mass of the Earth, or less than half the size of the moon. Sixteen asteroids have diameters of 240 kilometers or larger, the largest of which is Ceres, which has a diameter of about 1,000 kilometers.

Are all asteroids in our solar system in the main belt, or are there other bodies that share the space between Mars and Jupiter? And what about other asteroids belts out there? See the next page to go beyond the main belt.

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