Friday, May 7, 1999 Published at 14:51 GMT 15:51 UK

Rocky road around Earth

By BBC News Online Science Editor Dr David Whitehouse

There is probably a very thin belt of asteroids very close to the Earth. But the Oxford astronomers who have predicted the existence of the space rocks, after carrying out computer simulation work, say there is little to worry about - a collision with Earth is unlikely.

"These are good guys, so to speak," says Wyn Evans, who co-authored the research which appears in the journal Nature. "They're objects in stable orbits."

If the existence of the belt can now be proven by observation, it would be the fourth such ring of asteroids known in our Solar System.

The asteroid belt between Mars and Jupiter has been known for almost two centuries. Then researchers discovered the Kuiper belt, a region of stable orbits in the outer Solar System, beyond Neptune.

More recently they identified a region of orbital stability between Neptune and Uranus.

Few space rocks

Wyn Evans says the new belt has probably evaded detection so far because it contains relatively few asteroids.

"Unlike the main belt of the asteroids which lies between Jupiter and Mars, which has many tens of thousands of asteroids, there are probably no more than a 1,000 or so asteroids in this very thin belt which lies between Earth and Mars."

The computer work suggests the belt is between 10 and 40 million kilometres from the Earth. Some of the rocks within it could be more than 200 metres in diameter, the researchers say.

Although the rocks would lie in a very stable orbit around the Sun, Wyn Evans says there is always the possibility that they could be disturbed.

"There will be gravitational sculpting of the belt - in other words there will be some locations in the belt where asteroids will have been ejected via resonances, and these objects may well be on planet crossing orbits and could pose a threat."

The research also predicts another belt between the Sun and Mercury.