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Asteroids are rocky and metallic objects too small to be considered planets. They are sometimes called minor planets. They range in size from Ceres, with a diameter of about 1000 km, down to a few centimeters or less. The ajacent image shows the asteroid Gaspra, as photographed by the Galileo space probe (Ref). The name asteroids, meaning "star-like", derives from the fact that, compared with comets, they are star-like in appearance because since they are rocky they do not emit the gases and dust that give comets their fuzzy appearance.

Asteroids on a collision course with Earth are called meteoroids. If this meteoroid burns up because of frictional heating when it strikes our atmosphere, we term it a meteor (colloquially, a "shooting star"). If the meteoroid doesn't burn up completely and strikes the Earth we call it a meteorite. We shall discuss, meteoroids, meteors, and meteorites in the next section.
Asteroids represent material left over from the formation of the solar system. Although is has been suggested that asteroids are the remains of a planet that was destroyed in a massive collision, it is more likely that they represent material that never coalesced into a planet. If the estimated total mass of all asteroids was gathered into a single object, the object would be less than half the diameter of the Moon (Ref).

The Asteroid Belt
The highest concentration of asteroids is in a region lying between the orbits of Mars and Jupiter called the asteroid belt that is illustrated schematically in the adjacent figure. Here is a more realistic distribution of semimajor axes for the minor planets. Some 7000 asteroids have been identified so far. Most, but not all, have average orbital radii lying in the region of the asteroid belt. It is likely that the origin of the asteroid belt lies in the gravitational perturbation of Jupiter, which kept these planetisimals from coelescing into larger bodies.
Kirkwood Gaps
Asteroid orbit distributions show evidence for Kirkwood Gaps, which are certain orbital radii within the asteroid belt for which there are few asteroids. These gaps are associated with orbital radii that lead to orbital periods that are ratios of integer multiples of Jupiter's orbital radius. They result from resonance interactions with Jupiter that tend to eject asteroids from such orbits.
Galileo Flys by Gaspra
The adjacent figure shows the asteroid Gaspra, which was investigated by the Galileo spacecraft (Ref). Gaspra is irregular, with dimensions about 19 x 12 x 11 kilometers. This image was taken 10 minutes before closest approach on October 29, 1991, at a range of 5,300 kilometers.

The surface gravitational force is so weak on this small body that the gravitational escape speed is only 10 meters per second---comparable to the speed of a fast sprinter. In this gravitational field, a 200 pound man would weigh only 0.1 pounds!

Gaspra has a high concentration of small craters compared to larger ones relative to other small objects in the Solar System like the moons of Mars (which may be captured asteroids). This is consistent with the theory that Gaspra originated comparatively recently from the collisional breakup of a larger body perhaps 10 times the present size of Gaspra (Ref).
The Asteroid Ida and its Moon Dactyl
The Galileo spacecraft found a surprise when it flew by the asteroid Ida: Ida has a tiny moon, which has been named Dactyl! The adjacent color-enhanced picture shows both asteroid 243 Ida and Dactyl at about 14 minutes before closest approach of Galileo on August 28, 1993, at a range of 10,500 kilometers.

Ida (left) is about 56 kilometers long and Dactyl (right) is about 1.5 kilometers across in this view. Dactyl is actually in the foreground, about 80 kilometers closer to the spacecraft than Ida is. Dactyl is not identical in spectral properties to any area of Ida in view here, though its overall similarity in reflectance and general spectral type suggests that it is made of basically the same rock types (Ref).
The Spin of Asteroid Toutatis
By making a series of observations, it is possible to study the rotation of some asteroids. Most have simple rotations around a fixed axis, with periods typically between one hour and one day. For example, here is a movie (83 kB MPEG) made by the Hubble Space Telescope of the asteroid Vesta in rotation (Ref).

However, the asteroid 4179 Toutatis (which crosses Earth's orbit) has been found through radio telescope observations to have an irregular shape and a complex tumbling rotation---both thought to arise from a history of violent collisions. Here is a short animation (47 kB MPEG) of the spin of Toutatis; here is a longer animation (288 kB MPEG).

It is believed that violent collisions are common for asteroids, and that many asteroids have in their past experienced complex rotations like Toutatis as a consequence of such collisions. However, internal friction has caused them to dampen into simple rotation in relatively brief amounts of time; thus, most asteroids are observed to have simple rotations. But Toutatis rotates so slowly that this dampening process would take much longer than the age of the Solar System. Thus, the rotation of Toutatis may be a relic of the collisional evolution of an asteroid (Ref).

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