The Solar System and the Planet Earth 303
Asteroids
Asteroids are defined as small bodies that orbit around the Sun. They are
smaller than planets but larger than meteoroids where a meteoroid is an
object less than ten meters across making the distinction between an
asteroid and a meteoroid quite arbitrary. The largest asteroids like Ceres
in the Asteroid Belt and Haumea, Makemake and Eris in the Kuiper Belt
have been reclassified as dwarf planets. Again this distinction is again
quite arbitrary.
Most of the known rocky asteroids are found in the Asteroid Belt
between the orbits of Mars and Jupiter. They are irregularly shaped and
made of sulfate rocks. It is estimated that there are between one and two
million asteroids larger than 1 km in diameter and millions of smaller
ones. The asteroids in the Asteroid Belt rotate with periods ranging
from 3 to 20 hours. Over 90% of the asteroids have variable brightness
indicating the irregularity of their shape. The irregularity of their shape is
likely due to the collision of these objects with each other. A typical
example of the irregular shape of the asteroids is provided by Eros,
which passes close to the Earth. It is shaped like a brick with a length of
23 kilometers and a thickness and width of 8 kilometers each.
There is a great deal of variation in the orbits of the asteroids. While
a majority have nearly circular orbits a large number have highly
irregular orbits which brings one asteroid as close to the Sun as Mercury
and another as far from the Sun almost as Saturn. At the moment,
approximately 1600 asteroids have been discovered and identified. It is
estimated that there are as many as 100,000 asteroids altogether. The
total mass of all the asteroids in the Asteroid Belt is less than 0.2 of 1%
of the Earth’s mass, however.
It was originally believed that the asteroids once formed a planet,
which broke apart. The total mass of all the asteroids is too low to
make this theory credible. Instead it is believed that many asteroids were
formed at the same time with dimensions like Ceres and that the
fragmentation that has taken place since has been due to frequent
collisions.
In addition to the asteroids of the Asteroid Belt there are also
objects in the Kuiper Belt that behave very much like asteroids in that
they orbit the Sun in nearly circular orbits. They differ from the asteroids
in the Asteroid Belt between Mars and Jupiter in that they are made
of both rocks and ices of water, methane and ammonia. There is a