Asteroid Ida (NASA)

Chemical classification of asteroids

A first chemical classification of asteroids can be made thanks to spectral observations (you can click here to understand more about light and spectra). This classification finds also some correspondences with the meteorites classification, since there is a tight bound between these two classes of objects (asteroids and meteorites).
This classification must be taken very carefully, because of biases involved in the observations. For example, the experimental difficulty of detecting dark asteroids (e.g. the dark C-types ) could make the percentages of the different classes not representative of the true distribution of asteroids.

C-type ( where C stands for carbonaceous)

This class includes more than 75% of known asteroids. They are extremely dark (albedo 0.03), similar to carbonaceous chondrite meteorites. Approximately these asteroids have the same chemical composition as the Sun, minus hydrogen, helium and other volatiles. The spectra of these asteroids have relatively blue colors and are fairly flat and featureless.

S-type ( the S stands for silicaceous)

This type of asteroids represents about 17% of the totality. They are relatively bright objects (albedo .10-.22). They have a metallic composition (basically made of nickel, iron and magnesium-silicates).
The spectra relative to this class are reddish and similar to those of stony-iron meteorites.

The C and S types include most of the asteroids but there are a few smaller classes including :

M-type (M for metallic). This class includes most of the rest: they are bright asteroids (albedo .10-.18), made of pure nickel-iron.

E-Type (E for enstatite)

R-Type (R for red)

There is also a number of other very rare types that grows as more scientific information about asteroids is collected ....