General Asteroid Information

Asteroids are small metallic, rocky lumps that orbit the sun but are not large enough to be considered planets. Asteroids lack an atmosphere.

The majority of asteroids live in the main asteroid belt, a circular asteroid racetrack between Mars and Jupiter that is approximately two to four astronomical units (AU) from the sun. Most main belt asteroids have a stable, slightly elliptical orbit, usually taking between three and six years to circle the sun.

The theory on the existence of asteroids is that had the gravitational pull of Jupiter not been so strong, the asteroids within the asteroid belt would have formed one conglomerated mass and eventually into a planet with an orbit between Mars and Jupiter. It is estimated that the collective mass of all the asteroids is 930 miles (1,500 kilometers) in diameter, under half the size of the moon.

There are three major characterizations of asteroids, which are determined by composition, albedo, and similarity to meteoric samples:

Near-Earth Asteroids

Asteroids which come within 1.3 AU of the Sun are considered near-Earth asteroids. It is assumed that these asteroids have been knocked free from the main belt due to asteroid collisions or the gravitational influence of the planet Jupiter, or are the remnants of short-period comets. The largest known near-Earth asteroid is Ganymed, with an approximate diameter of 25.5 miles (41 kilometers).

There are three main characterizations of near-Earth Asteroids, named for famous examples of each:

Interesting Stuff

Several asteroids are confirmed to have moons orbiting them, including Ida (which the spacecraft Galileo took photographs of in a 1993 fly-by -- the moon was named Dactyl), and Eugenia (which was confirmed by the Canada-France-Hawaii Telescope on Mauna Kea in 1999). This discovery allows observers to make more concise estimates of mass because of the gravitational relationship between the primary asteroid and its moon. These new observations have shown that many asteroids are far less dense than previously thought.