The Asteroid Belt
Asteroids are rocky and metallic objects too small
to be considered planets. They are sometimes called
minor planets. They range in size from Ceres, with a
diameter of about 1000 km,
down to a few centimeters or less.
The asteroids with diameters larger than 300 km are listed in the
table below.
Location of Asteroids
The highest concentration of asteroids is in a region lying between the
orbits of Mars and Jupiter called the asteroid belt. It is
illustrated schematically in the adjacent figure.
Contrary to popular depictions in some movies, the
asteroid belt is mostly empty space. There is a higher concentration of asteroids there than
anywhere else, but your chances of even seeing an asteroid of significant size
if you flew through the asteroid belt on a
random path are small.
Here is an
applet illustrating some asteroid orbits.
|
Largest Asteroids
|
| Asteroid |
Diameter (km)
|
| Ceres |
960
|
| Pallas |
608
|
| Vesta |
555
|
| Hygiea |
450
|
| Euphrosyne |
370
|
| Interamnia |
350
|
| Davida |
323
|
| Cybele |
309
|
|
|
|
|
|
Apollos, Amors, and Trojans
The above figure also illustrates two groups of asteroids not restricted to the asteroid belt:
the Apollo asteroids, which have elliptical orbits crossing that of the Earth,
and the Trojan asteroids, which are trapped at points rotating before and after
Jupiter on its
orbit. Another group of asteroids not depicted on the figure that are not
confined to the asteroid
belt are the Amor asteroids, which have orbits crossing the orbit of Mars but not Earth.
Number of Asteroids
Some 400,000 asteroids have been
identified so far but it is estimated that there are at least 500,000 asteroids of diameter
1 kilometer or more in the Solar System. Although their number is relatively large, the
total volume occupied by asteroids is not.
If we were to collect the estimated total
mass of all asteroids into a single object, the object would be
much smaller than the Moon.
Most, but not all, of these asteroids have average orbital radii (length of the semimajor
axis for the orbit) lying in the
region of the asteroid belt. Approximately 2000 known asteroids (for example,
the Apollo asteroids)
have orbits that take them into the inner
Solar System.
Origin of the Asteroid Belt
At one time it was thought by some that the asteroid belt represented a planet that had broken apart.
Instead, it is likely that the origin of the asteroid belt
lies in the gravitational perturbation of Jupiter, which kept these
planetesimals from ever condensing into larger bodies.