Photograph by Russel Kempton
April 19, 1995, NASA
The majority of asteroids are contained within a main belt that exists between the orbits of Mars and Jupiter, roughly 2-4 AU form the Sun. An AU, or astronomical unit, equals 149,597,870 km (approximately 92,750,679 miles), the average distance from the Earth to the Sun. There are, however, a substantial number of asteroids that lie in orbits that bring them closer to Earth. These are the "near-Earth asteroids," or NEAs. It is believed that these NEAs could be pieces cast out of the main asteroid belt by the gravity of Jupiter and may become candidates for the origins of meteorites. Asteroids should not, however, be confused with meteors or comets, for that matter.
A meteoroid is a small object (below 1 km in diameter) in an independent orbit in the solar system. Generally a meteoroid is a piece of a comet or asteroid. When a meteoroid strikes our atmosphere at high velocity, friction causes this piece of rocky matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what is left strikes the Earth's surface and is called a meteorite. Much of the understanding of asteroids comes from studying meteorites.
Not only are asteroids different from meteors and comets, but they are also different from each other in various ways. Asteroids are classified into different "types" according to their albedo and the spectrum of sunlight reflected off their surfaces. The reflected spectrum contains "fingerprints" of minerals on the asteroids, which can be used to connect them with classes of meteorites containing the same minerals. Albedo is a measure of an object's reflectivity. For instance, a white, perfectly reflecting surface would have an albedo of 1.0. A black, perfectly absorbing surface would have an albedo of 0.0. The following classification is just one of several schemes used today to categorize asteroids. This classifications' letters were chosen as mnemonics for similarities seen with carbonaceous, stony, and metallic meteorites.
C-type: This type includes more than 75% of known asteroids. It is extremely dark with an albedo of 0.03-0.09. C-type asteroids are thought mostly to consist of a type of meteorite called "carbonaceous chondrites," a mixture of rock and tar that has a composition much like the sun, minus the hydrogen and helium.
S-type: This type includes about 17% of known asteroids. It is relatively bright with an albedo of 0.10-0.22. Its composition is metallic nickel-iron mixed with iron- and magnesium-silicates.
M-type: This type includes most of the rest of the known asteroids. It is also bright with an albedo of 0.10-0.18. Most are though to consist of nickel-iron with a small amount of iron- and magnesium-silicates.
There are a dozen or so other rare types, but nearly all are variants of the three major ones.
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