Asteroid types

Asteroids are very different in shape and size (ranging from a few kilometres to more than 100 kilometres), as well as in their composition. Rosetta will provide information on the mass and density of the asteroids that it approaches, thus telling us more about their bulk composition. It will also measure their subsurface temperature and look for gases and dust around them.

Based on their albedo, or refelectivity (a white, perfectly reflecting surface has an albedo of 1 whereas a black, perfectly absorbing surface has an albedo of 0), spectral characteristics and similarities to known types of meteorites, asteroids are classified into at least 14 different classes, whereby the following three major types occur:

C-type (carbonaceous)

• More than 75% of known asteroids
• Very dark, with an albedo of 0.03-0.09
• Similar to carbonaceous chondrite meteorites
• Composition is thought to be similar to the Sun, depleted in Hydrogen, helium, and other volatiles
• Inhabit the main belt's outer regions

S-type (silicaceous)

• About 17% of known asteroids
• Relatively bright with an albedo of 0.10-0.22
• Composition is metallic iron mixed with iron- and magnesium-silicates.
• Dominate the inner asteroid belt.

M-type (metallic)

• Includes most of the rest of the known asteroids
• Relatively bright, albedo of 0.10-0.18
• Pure nickel-iron composition
• M-type asteroids inhabit the main belt's middle region

In addition to these major types there are several less common types of asteroids.

Steins is an E-type asteroid, which is defined by a high albedo and a featureless spectrum. They resemble the characteristics of the Aubrite meteorites and are believed to have suffered at least partial melting. Steins therefore may be a fragment of a larger asteroid that fragmented a long time ago.

* arrival 23 December 1999, touch down 12 February 2001
**12 November 2005 (touch down for sampling)