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.



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.




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.




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.