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Asteroids are chunks of rock left over from the formation of the Solar System. There are millions of these, most of which live in the asteroid belt between Mars and Jupiter. They have sizes from a few centimetres across to a few kilometres.
Many asteroids are in orbits that take them close to the Earth. 312 of these are classed as "potentially hazardous" which means that they will pass very close, but none of these are going to hit the Earth in the next hundred years.
Asteroids spin once every few hours. Asteroids can also spin in all directions, so seem to tumble through space.
NASA recently sent a spacecraft called Near Earth Asteroid Rendezvous (NEAR) spacecraft to visit one of these asteroids, called Eros, to try and learn more about it. When NEAR had done all of the experiments it was supposed to do, NASA landed it on the surface of Eros. There are several other missions being planned by NASA, the European Space Agency (ESA) and the Japanese Space Agency. These are all due for launch sometime in the next twenty years.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001