Our Solar System .... Asteroids.
Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets.
Asteroids range in size from the dwarf planet 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.
Some of the best preserved examples are the Barringer Meteor Crater near Winslow, Arizona, or the Wolf Creek Crater in
South Australia. Asteroids are material left over from the formation of the solar system. One theory suggests that they
are the remains of a planet that was destroyed in a massive collision long ago.
More likely, asteroids are material that
never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object,
the object would be less than 1,500 kilometres (932 miles) across -- less than half the diameter of our Moon.
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth.
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's 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 through 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.
On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde.
This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. This visit was unique because
NEAR was not designed for flyby encounters. NEAR was an orbiter destined for asteroid Eros in January of 1999.
Astronomers have studied a number of asteroids through Earth-based observations.