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Wednesday, 4 October, 2000, 22:44 GMT 23:44 UK
Tiny force nudges space rocks our way
Rock Nasa
Chips off asteroids can head towards Earth
By BBC News Online science editor Dr David Whitehouse

The reason why so many space rocks rain down on Earth is due the uneven heating they experience from the Sun when they are still millions of kilometres away from our planet.

Almost all meteorites are chipped off asteroids during collisions, but the way this debris manages to reach us has always been a puzzle.

Rock JPL
A radar impage of asteroid Kleo
The answer, according to David Vokrouhlický, of Charles University in Prague, Czech Republic, and the late Paolo Farinella, of the University of Trieste, Italy, depends upon a tiny force that acts on the rocks as they move through space.

The force, caused by Sun heating up just one side of the rocks, steers the debris into one of two particular orbits where the gravity of Jupiter and Saturn will then fling it towards the Earth.

Russian engineer

Nearly all the asteroids reside in a belt between the orbits of Mars and Jupiter, about 330 million to 480 million kilometres from the Sun.

It was thought fragments from asteroid collisions only escaped the belt if they passed into one of two narrow regions called resonance orbits, where Jupiter and Saturn's gravity could hurl them towards the inner Solar System.

But there is a big problem with this idea. There are not enough asteroids near the resonances to explain the number of fragments that enter the Earth's atmosphere. About 1,000 tonnes of rock strikes the Earth each year and ends up as meteorites on the ground.

Writing in the journal Nature, the researchers suggest an explanation can be found in a tiny effect first described a century ago by the Russian engineer Yarkovsky. He said that a rotating body in space that was warmed by the Sun would emit heat unevenly.

This results in a small force that causes asteroids to drift from their original orbits, directing more material than expected towards the resonance ejection point. Most will fall into the Sun, but a surprisingly large 0.5% will reach the Earth.

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