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Last Updated: Thursday, 8 May, 2003, 08:02 GMT 09:02 UK
Meteor's family poses puzzle
By Dr David Whitehouse
BBC News Online science editor

Researchers have analysed the path of a fireball that exploded over central Europe in 2002, and shown the space rock responsible came from an almost identical orbit to that of a meteorite which fell to Earth in 1959.

Fireball April 2002, Nature
The fireball was caught on camera

But although the trajectories about the Sun of the two rocks appear the same, the meteorites themselves are dissimilar in composition.

This is leading researchers to reassess their understanding of how meteoroids can group into streams as they circle our star.

Such streams may contain a more varied collection of rocks than previously believed.

Brighter than the Moon

The 2002 fall was of an object about 300 kilograms in mass that had been orbiting the Sun between the Earth and the asteroid belt for millions of years.

It came down to Earth on 6 April 2002.

As the rock plunged into the atmosphere, it became a very bright fireball as it passed over western Austria and southern Bavaria.

Far brighter than the full Moon, the fireball was seen by people scattered all over central Europe. Residents in towns and villages reported shaking ground, rattling windows and sounds coming from the sky.

But in addition to these casual observations, the exploding meteor's (or bolide's) trajectory was recorded by cameras and various other sensors, making it unusually well documented for such an event.

Dark and cooling

The data indicate the fireball's luminous trajectory was about 91 km long, starting at an altitude of 85 km about 10 km east-northeast of Innsbruck, Austria, terminating at an altitude of 16 km, about 20 km east of Garmisch-Partenkirchen, Germany.

It entered the atmosphere at a speed of 21 km per second and decelerated to 2.4 km per second, by which time the ablation of its surface by atmospheric friction ceased, and it fell to Earth dark and cooling.

Recovered fragment, Nature
A fragment was recovered

Researchers, from the Czech Republic and Germany, identified a "footprint" about 800 metres wide and several km long as the most probable location for any fragments from the bolide to be found.

Remarkably, a piece was recovered in a mountainous area on 14 July.

And then the story became more intriguing.

Non-identical twins

The data on the object's fall enabled its orbit around the Sun to be determined. It is rare to have a physical specimen and an accurate orbit; there are but a handful of examples.

Curiously, the meteorite had come out of an almost identical orbit to that of a space rock which fell to Earth in 1959. It, too, had extensive data available about it. "This paired meteorite fall is probably not a coincidence," say the researchers.

But although the two meteorites might be from the same stream orbiting the Sun, they are not identical. Their compositions and the time they had spent wandering space are different.

The effects of cosmic rays on rocks in space enable astronomers to determine roughly how long an object has been in orbit. The 2002 meteorite had been in space for 48 million years but the 1959 object only 12 million years.

The tentative explanation proposed by the researchers is that streams of meteoroids may be more varied than was previously thought, as it seems unlikely that both rocks came from the same parent body.

The research is published in the journal Nature.

'Meteor' lights up Midlands
06 Oct 02  |  England
Shooting star observed from space
17 Feb 00  |  Science/Nature

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