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 Wednesday, 20 November, 2002, 22:02 GMT
Asteroid threat reassessed

Every year a small asteroid explodes in the Earth's atmosphere with an energy equivalent to 5,000 tonnes of TNT, according to new information.

The assessment comes from researchers who have studied about 300 impacts from space observed by US military surveillance satellites.

The scientists now estimate an object of the size that exploded over central Siberia in 1908 causing widespread devastation only strikes the Earth every 1,000 years or so. This is far less frequent than had been thought.

The asteroid impact assessment has been published in the journal Nature.

Military data

Asteroids with diameters smaller than 50-100 metres that collide with the Earth usually do not hit the ground as a single body. Rather, they detonate in the upper atmosphere.

A new analysis of the flashes of light from these exploding asteroids is possible because of data provided by the US Department of Defense from military satellites.

Positioned in geostationary orbit, the satellites have a view of a large part of the Earth and, because they are designed to detect light from rockets being launched, they are able to see the light flashes from space impacts as well.

Between February 1994 and September 2002, about 300 impact events were seen. From the intensity and duration of the light flashes, and some basic physics, it was possible to calculate the size of the incoming asteroids.

The researchers led by Dr Peter Brown, of the University of Western Ontario, Canada, estimate that every month an object explodes in the upper atmosphere with an energy equivalent to 300 tonnes of TNT.

'Harmful intruders'

Every 10 years, an object with the energy of 50 kilotonnes impacts the Earth.

An object like the one that struck Tunguska in central Siberia in 1908 hits us, on average, every 1,000 years or so. That object had an energy equivalent to 10 megatonnes of TNT. If such an object were ever to strike an inhabited area, millions of people could be killed.

Recently scientists have expressed concern that upper atmosphere explosions caused by small asteroids could be mistaken for nuclear detonations, especially during times of international tension.

Surveying the latest data, Dr Benny Peiser, of Liverpool John Moores University, UK, told BBC News Online: "This new research reinforces our view that we are constantly bombarded by cosmic debris large enough to be misinterpreted as a nuclear attack.

"The findings are a compelling warning that we need to start scanning the skies for small, but potentially harmful intruders."

And commenting in the journal Nature, Dr Robert Jedicke, of the University of Arizona, US, says the study has "linked the fields of meteor and comet/asteroid planetary astronomy in a manner which shows that they are not merely distant cousins."

See also:

15 Nov 02 | Science/Nature
01 Aug 02 | Science/Nature
29 Jul 02 | Science/Nature
08 Nov 01 | Science/Nature
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