Classification of Asteroids

Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo:

• C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;
• S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;
• M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron.
• There are also a dozen or so other rare types.

Because of biases involved in the observations (e.g. the dark C-types are harder to see), the percentages above may not be representative of the true distribution of asteroids. (There are actually several classification schemes in use today.)

There is little data about the densities of asteroids. But by sensing the Doppler effect on radio waves returning to Earth from NEAR owing to the (very slight) gravitational tug between asteroid and spacecraft, Mathilde's mass could be estimated. Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a compacted pile of debris.

Asteroids are also categorized by their position in the solar system:

• Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).
• Near-Earth Asteroids (NEAs): ones that closely approach the Earth
  • Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;
  • Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU
  • Amors: perihelion distances between 1.017 and 1.3 AU;
• Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)

Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps. These are regions where an object's orbital period would be a simple fraction of that of Jupiter. An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit.