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.