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Forget Bruce Willis, Earth's atmosphere is our best defense against meteorites

Porous space rocks just can't handle the pressure

Researchers have discovered why most meteorites disintegrate before they reach Earth - and it's all to do with atmospheric pressure.

Previously it was thought that compression of air particles is what causes meteorites to wear away as they make their way down to our planet's surface, but many, like the 2013 Chelyabinsk meteorite that injured over 1,000 people, explode in the atmosphere. New research suggests that air is forcing its way inside the meteorite itself.

The research, published in the journal Meteoritics & Planetary Science, notes that in the last few years of exploration we've discovered that many meteorites aren't solid boulders but instead loose conglomerations of rocks that have glommed together through gravitational attraction.

Porosity rates of up to 20 per cent have been found in some meteorites and this could spell their doom once they hit Earth. As the rocks hit the lower, denser atmosphere air is forced inside the porous meteorites and heats up massively, so that the pressure increase eventually blows the object into tiny fragments.

"There's more going on than what had been thought before," said Jay Melosh, a geophysicist at Purdue University in Indiana. "Bottom line is that the atmosphere is a better screen against small impacts than we had thought."

Doesn't matter

For example, the original Chelyabinsk meteorite would have weighed in at around 12,000 tons. However, scientists were only able to find around five tons of it. While air compression will have burnt or battered material off the meteorite the explosion when it was above the surface may have destroyed a lot more matter.

Melosh and his team modeled the destruction of meteorites using software first developed by Los Alamos National Laboratory to model nuclear reactor explosions. The simulations showed a dense patch of air formed in front of the meteorite, while behind it there was a near vacuum.

This pressure differential forces air through the porous meteorite and the air compression from the body means the air is superheated. The ensuing air expansion can be an explosive combination.

The simulations showed that porous meteorites start to fragment around 46 kilometers (29 miles) up and explosively disassemble at an altitude of around 39 kilometers (24 miles). By contrast, meteorites with low porosity were still going strong until they hit the 36 kilometer (22 miles) up.

This doesn't mean that we can bank on a massive civilization-changing meteorite breaking up before it hits Earth. But it does give hope that smaller objects might not be as devastating as first thought. So no need for a team of maverick offshore drilling grunts. ®

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