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.