Asteroids are rocky and metallic objects that orbit the Sun but are
too small to be considered planets. They are known as minor planets.
Asteroids range in size from Ceres, which has a diameter of about 1000 km,
down to the size of pebbles. Sixteen asteroids have a diameter of 240 km
or greater. They have been found inside Earth’s orbit to beyond Saturn’s
orbit. Most, however, are contained within a main belt that exists between
the orbits of Mars and Jupiter. Some have orbits that cross Earth’s path
and some have even hit the Earth in times past. One of the best-preserved
examples is Barringer Meteor Crater near Winslow, Arizona.
Asteroids are material left over from the formation of the solar system.
It was once believed that the asteroids were the debris of a large planet,
orbiting between Mars and Jupiter, which had suffered a major catastrophe
and fragmented into many smaller parts. However, it is now thought that no
planet could ever have formed in this zone, because the strong
gravitational influence of the newly formed Jupiter would have prevented
the smaller planet from forming. Instead, the “building blocks” of rock,
built up through collisions with the smaller particles present at the
formation of the solar system, were left, and are what we call the
asteroids today. Much of our understanding about asteroids comes from
examining pieces of space debris that fall to the surface of Earth.
Because the asteroids orbit in the gap between Mars and Jupiter, it is not
surprising that the massive planet affects them. Astronomers in the 1800’s
noticed that the asteroid belt has gaps in it, particularly at distances of
2.5 and 3.28 astronomical units from the Sun. The astronomer Daniel
Kirkwood explained these gaps by considering the orbit, which a body at
this distance would have. He discovered that any asteroids in these gaps
would be lined up with Jupiter very often, and so it would be pulled by the
gravitational influence of the planet, out of the gap. For this reason,
these are now called the Kirkwood gaps, and now there are several known.
This, however, is not the only effect, which the largest planet in our
solar system has on these small objects.
Asteroids that are on a collision course with Earth are called meteoroids.
When a meteoroid strikes our atmosphere at high velocity, friction causes
this chunk of space matter to incinerate in a streak of light known as a
meteor. If the meteoroid does not burn up completely, what is left strikes
Earth’s surface and is called a meteorite. Of all the meteorites examined,
92.8 percent are composed of silicate (stone), and 5.7 percent are composed
of iron and nickel; the rest are a mixture of the three materials. Stony
meteorites are the hardest to identify since they look very much like
terrestrial rocks. Because asteroids are material from the very early
solar system, scientists are interested in their composition. Spacecraft
that have flown through the asteroid belt have found that the belt is
really quite empty and that asteroids are separated by very large
distances. Before 1991, the only information obtained on asteroids was
though Earth based observations. Then on October 1991 asteroid 951 Gaspra
was visited by the Galileo spacecraft and became the first asteroid to have
hi-resolution images taken of it. Again on August 1993 Galileo made a
close encounter with asteroid 243 Ida. This was the second asteroid to be
visited by spacecraft. Both Gaspra and Ida are classified as S-type
asteroids composed of metal-rich silicates.
Throughout the history of the solar system, the Earth and other
planets have been subjected to impacts from smaller bodies such as comets
and asteroids and sometimes with catastrophic consequences. Learning and
trying to understand these space objects can help us identify the
composition of the universe and maybe find out how it started.