Asteroids are small, usually irregularly shaped bodies orbiting the sun, most often at least partially between the orbits of Mars and Jupiter. Ceres is the largest asteroid (diameter: 470 mi/750 km) and was the first discovered (1801). Of the more than 2,000 asteroids known, most have been discovered photographically; their paths appear as short lines in a time exposure. Asteroids may be fragments of a planet shattered in the remote past; material that failed to condense into a single planet; or material from the nuclei of old comets. The Trojan asteroids revolve in the same orbit as Jupiter, kept by perturbation effects in two groups 60° ahead of and 60° behind Jupiter. A Trojan asteroid has also been found in Mars' orbit. Some asteroids, such as the Apollo asteroids, cross the earth's orbit; they may be the cause of the earth's several meteorite craters and are a possible location for future mining. The space probe Galileo, which passed near and photographed Gaspra (1991) and Ida (1993), provided the first close images of an asteroid. The pictures revealed that Ida has a natural satellite, Dactyl.

     What if an asteroid was in earth's path? It's simple, we'd be obliterated unless the asteroid was small, but anything over the size of a building would be devastating. The Earth gets hit daily by extremely small asteroids or space dust. Some well-known impact sites are in Arizona and Siberia. If you're looking for some good movies to see about asteroids try Armageddon or Deep Impact.