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 THE
ASTEROID BELT
Asteroids are scattered across a vast region of space between
Mars and Jupiter known as the Asteroid Belt.
In science fiction movies, it often seems as if the asteroid
belt is narrow and that there are asteroids everywhere, so that
space ships have to dodge around them. In fact, the chances
of even seeing an asteroid, if you flew through this part of
space, are small. The asteroid belt is about 300 million kilometers
wide and the total weight of all the asteroids is much less
than the Moon. Most asteroids are less than 1 km across. The
first asteroid, Ceres, was discovered in 1801 by the Italian
astronomer Giuseppe Piazzi. Thousands of new asteroids are still
being discovered each year as our telescopes continue to improve.
Within the asteroid belt, there are zones where many asteroids
are concentrated, and zones that are quite empty. The empty
spaces have been created by tidal forces from neighboring Jupiter.
A similar mechanism creates gaps in the rings around Saturn.
Rocks that are thrown out of the spaces in the asteroid belt
by tidal forces are likely to become meteorites that land on
Earth.
| [pictured
above] The asteroid Ida was imaged in 1993 by the
Galileo spacecraft, as it passed by on its way to Jupiter.
Ida is 58 km long and 23 km across. Ida has a small satellite
(moon) named Dactyl, which is the small white dot on the
right of the picture. Dactyl, only 1.6 km across, circles
around Ida at a distance of 90 km. |
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 WHAT
ARE ASTEROIDS MADE OF?
Not all asteroids are alike: they are different colors on
the surface and are made of different kinds of materials.
We can learn a lot about what asteroids are made of by studying
the sunlight that is reflected off their surfaces. Each mineral
has a certain way of absorbing (soaking up) light, and we can
see a mineral's fingerprint in the light that has bounced off
the asteroid's surface. We call this technique reflectance spectroscopy.
Some asteroids have metal surfaces. Some are rocky and made of
basalt. Some are a mixture of rock 
minerals and metal. Some are very icy and have a lot of organic
material. The different kinds of asteroids match the different
materials we see in meteorites.
 Reflectance
spectroscopy can only measure the composition of the very top
layer of material on the surface of an asteroid. It does not tell
us anything about what the rock is like if we dig down. Because
mineral surfaces change when they are exposed to radiation in
space, we can't look at the fresh rock composition. Comparing
reflectance data
with meteorites helps to address some of the problems with "space
weathering."
[pictured
top] The asteroid Eros became the first asteroid
on which a spacecraft landed, when the NEAR spacecraft touched
down on Eros in 2001. Eros is 33 km long and 13 km wide.
It is known as an S-type asteroid. From the way the S-type
asteroids reflect light we think that they are made of material
like ordinary chondrites.
[pictured
center] The reflectance spectrum of asteroid Eros
shows that the minerals olivine and pyroxene are present
on the surface. Both of these minerals occur in ordinary
chondrites.
[pictured
above] The Correo ordinary chondrite. Asteroid Eros
is thought to be made out of rock like this meteorite. Ordinary
chondrites contain the minerals olivine, pyroxene, iron-nickel
metal and iron sulfide. |
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 WHICH
ASTEROIDS DO METEORITES COME FROM?
We know that meteorites come from asteroids, but we do not
know exactly which asteroid each meteorite comes from.
If the fall of a meteorite is witnessed carefully, it is possible
to calculate its orbit. The orbits of several meteorites show
beyond a doubt that they originated in the asteroid belt. But
the orbits are not known accurately enough to be able to pinpoint
a certain asteroid.
 Using
reflectance spectroscopy, we can match different types of meteorites
to different types of asteroids. But it is hard to be more certain
about an exact match. The best match we can  make
at the moment is between basaltic meteorites and the asteroid
Vesta. We think that ordinary chondrites probably come from S-type
asteroids like Eros. However, when the NEAR spacecraft visited
Eros in 1999 it found that there was not enough sulfur on the
surface for it to be an exact match with an ordinary chondrite.
Maybe the sulfur on the surface of Eros has been lost to space?
Scientists are trying to answer this puzzle.
[pictured
top] From Earth, most asteroids are blurry objects
even in the best telescopes. This image of asteroid Vesta
was taken by the Keck Telescope in Hawaii. At 530 km across,
Vesta is the third largest asteroid. It was discovered in
1807 by the German astronomer H. W. Olbers.
[pictured
above right] We think that the "HED" group
of achondrite meteorites comes from Vesta. The way the light
reflects off Vesta tells us that the same minerals are on
the surface as the ones in the HED meteorites. The HED meteorites,
like Millbillillie in this image, are basalts. Vesta once
had volcanoes on its surface!
[pictured
above] Thin section image of the Pasamonte eucrite
(achondrite). The brightly colored grains are pyroxene and
the thin, grey grains are plagioclase feldspar. This rock
is a basalt. The image is taken in cross-polarized light
and is 3 mm across. |
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 THE
PLANET THAT NEVER WAS
The asteroid belt is a place where a planet might have grown.
But because it is so close to massive Jupiter, a large planet
could not form in this part of the Solar System.
The asteroid belt was discovered when astronomers searched the
space between Mars and Jupiter for a planet that they thought
should have formed there. Instead of one large planet, they found
millions of small asteroids. At one time, it was thought that
the asteroid belt might be the remains of an exploded planet.
But from our studies of meteorites we can tell that this was not
the case. A planet never grew in that part of the Solar System
because of the powerful gravity from nearby Jupiter.
A large planet would have been big enough to get very hot from
the decay of radioactive elements. Geological processes would
have melted and mixed the rock of the entire planet. This clearly
did not happen to meteorites. For example, the record of the formation
of the Solar System in chondrites would have been obliterated.
The biggest asteroids that ever grew were about the same size
as the biggest we see today, around 1000 km across.
| [pictured
above] Instead of a large planet between Mars and Jupiter,
there are thousands of asteroids. The biggest asteroid, Ceres,
is 466 km across. This is much smaller than the smallest planet,
Mercury, which is 2439 km across. In 2006, Ceres was named
a "dwarf planet," along with Pluto. |
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