Asteroids are small rocky objects left over from the formation of the Solar System. They are found mainly in the asteroid belt between Mars and Jupiter and range in size from a few cm to a few km. So far probably about 99% of asteroids over 100 km have been found, but there are many millions of smaller ones that are being discovered all the time.
Many asteroids are in orbits that cross the Earth’s. 312 of these are classed as potentially hazardous which means that they will pass within 0.05 AU of the Earth’s orbit, but none of these are likely to hit us within the next hundred years.
Asteroids have orbits with higher eccentricities and inclinations than the major planets and rotational periods ranging between a few hours and several weeks, but usually between 6 and 24 hours. Asteroids can also spin in all three axes, making them appear to tumble through space.
The most recent mission to an asteroid involved NASA’s Near Earth Asteroid Rendezvous (NEAR) spacecraft, which touched down on the surface of Eros at the end of its operational lifetime. 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.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001