Beyond size, shape and rotation, we know relatively little about these objects. Estimating their mass is difficult because they are not large enough to perturb the gravity of Mars or Jupiter, but Ceres is thought to be about 2.6 billion trillion pounds (1.2 x 1021 kg). Their densities are about 2 to 4 g/cm3, which is typical of rocky bodies. By examining the spectra of light reflected from these objects, we can classify asteroids as follows:
- C - Dark, probably carbon-containing (carbonaceous)
- S - Twice as bright as C, probably made of stony iron
- M - Similar to iron meteorites
- P and D - Low brightness, reddish
Asteroids appear to be of two different origins:
- Primitive, essentially unchanged pieces of the early solar system (C)
- Smashed remnants of differentiated pieces of the solar system
We think that asteroids are the remainders of planetismals, early pieces of the solar system, that formed between Mars and Jupiter. Some of the planetismals began to form into planets, but were smashed apart by Jupiter's immense gravity. Others did not begin to form planets (for unknown reasons).
Many questions remain about asteroids, because we have never been able to study them closely. Until now.