Asteroids are small, rocky bodies that orbit the sun in between the orbits of Mars and Jupiter, which is anywhere from 2.1 AU (1.95×108 mi / 3.15×108 km) to 3.2 AU (3.0×108 mi / 4.8×108 km) from the sun. There are more than 20,000 known asteroids. They are irregularly shaped and vary in size from a radius of 1 km (0.62 mi) to several hundred kilometers (Ceres is the largest, with a radius of 284 miles / 457 km). By measuring fluctuations in their brightness, we know that many asteroids rotate in periods of three to 30 days.

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


Photo courtesy NASA
Asteroid Gaspra, as viewed from the Galileo spacecraft