ESA Science & Technology05-Jul-2005 16:49:13

Asteroid Encounters

Where Are Asteroids Found?

About 70 000 asteroids have so far been discovered and given permanent numbers, but there are thought to be billions more waiting to be found. Nearly all of these are contained within a doughnut-shaped region between the orbits of Mars and Jupiter. On the inner edge of this main belt, asteroids take about three years to orbit the Sun, while those near the outer limit of the main belt take twice as long.

The main belt is thought to exist because of the strong gravitational influence of the giant planet, Jupiter. At a time when 'protoplanets' were colliding and growing, Jupiter prevented the formation of a planet.

Although there are billions of asteroids, the region they occupy is so vast that most of them are separated by thousands of kilometres of empty space. However, since they usually follow elliptical orbits, collisions are fairly common.

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