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Wednesday, 31 January, 2001, 22:22 GMT
Comets culled in carnage
What happens when a comet breaks up
By BBC News Online science editor Dr David Whitehouse

New calculations suggest the vast and distant cloud of comets that encircles the Sun may be more tenuous than had been thought.

Many comets are believed to come from a region of space billions of kilometres away, in a sphere of space beyond the outermost planet of Pluto but not as far as the nearest star.

This Oort cloud, as it has become known, was probably formed when small icy bodies forming in the early Solar System were ejected by the gravity of the much bigger giant planets, Jupiter, Saturn, Uranus and Neptune.

But a new study suggests that before the icy bodies were flung out, many of them would have suffered catastrophic collisions. The result was that far fewer comets made it to the Oort cloud.

Rock and ice

Four and a half billion years ago, the planets were formed from the accumulation of grains of dust, ice and gas that swirled around the young Sun.

Oort object Nasa
Objects in the Oort cloud survived a violent past
A few hundred million kilometres from the Sun, a series of large planets formed that were initially surrounded by a swarm of smaller bodies made of rock and ice - the comets.

In previous computer simulations of the early Solar System, these comets were gradually flung out into deeper space when they strayed too close to one of the giants.

But, according to Alan Stern of the South Western Research Institute in Colorado and Paul Weissman of the Jet Propulsion Laboratory in California, US, another factor should be taken into account.

Big space

"Physical collisions between comets play a fundamental and hitherto unexpected role during the ejection process," the researchers say.

They propose that most of the newborn comets did not travel very far before they were destroyed by impacts with other comets. Indeed, it is quite likely that the collisions prevented most of the comets from escaping.

But any objects that survived the cometary carnage are now safe. Today, collisions among the objects beyond Pluto must be very rare.

Although there are probably many millions of objects roaming the dark, outer reaches of the Solar System, the spaces they move through are so vast that they never encounter each other.

The research is published in the journal Nature.

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13 Oct 99 | Sci/Tech
A planet beyond Pluto
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