Asteroids are the remnants from the formation of the terrestrial planets. Compositionally, they are classified according to their reflection spectra. The three main types are M (metals: typically iron and nickel), S (stony), and C (carbonaceous). There are many subcategories and intermediate types.

Structurally, asteroids can be monolithic, fractured, or loose assemblages of rocks (referred to as "rubble piles" because they are held together only by their own gravity).  Their densities appear to be less than what would be expected from similar materials on Earth.  Asteroids larger than about 200 m appear to be "rubble piles" because they have spin periods smaller than the critical value at which centrifugal force would cause them to fly apart.  It appears that the spin period of smaller asteroids is not limited.  Therefore, they could be monolithic objects.

Southwest Research Institute: Space Instrumentation Division To report any problems, email: Walter Huebner
Last Updated: 3/12/2009