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On 30 June 1908, a bolide streaked across the skies above Lake Baikal near the border of Russia and Mongolia. Seconds later, a huge explosion above the taiga some 600 kilometres to the northeast flattened an area of forest the size of Luxembourg and went on to scorch trees for hundreds of kilometres around.

The detonation took place in a more or less uninhabited part of Russia called Tunguska but the explosion lit up skies across the northern hemisphere for three nights, interfered with the Earth’s magnetic field and triggered strong seismic and acoustic waves that shook the entire planet.

Despite a century of study, many aspects of the Tunguska event are still unexplained. For example, the explosion released more energy than a thousand Hiroshima-type atom bombs and yet left no crater. A similar-sized object is thought to have hit North America some 12 000 years ago, triggering the megafaunal extinction and widespread cooling. And yet the Tunguska event seems to have left our climate intact.

Now a new analysis by Edward Drobyshevski of the Russian Academy of Sciences in St Petersburg Russia claims to have solved these problems. Drobyshevski concludes that the object that hit in 1908 was a comet (as have many scientists before him). But unlike the others, he has been able to calculate that this comet hit the Earth’s atmosphere almost at a tangent and broke apart.

The larger part of this comet skipped off the atmosphere, back into an Earth-crossing orbit (we should expect to find it nearby, predicts Drobyshevski).The smaller part rapidly heated up above Russia before exploding in the atmosphere over Tunguska.

The key to why it left so little lasting damage is the nature of the explosion, says Drobyshevski. And the key to that is our improved understanding of the chemical make up of comets. He says the comet would have had a high hydrogen peroxide content and this would have dissociated explosively as it heated up to produce oxygen and water, breaking the comet apart. It was this explosion that devastated Tunguska.

“Significantly, the energy of the chemical explosion is substantially lower than the kinetic energy of the body,” says Drobyshevski.

This explains the comet’s relatively benign affect on the planet and solves many of mysteries associated with the event, he says.

Interesting idea–why should all cometary impacts be head on affairs? If correct, it looks like we’ve had a lucky escape, this time.

Ref: arxiv.org/abs/0903.3309: Tunguska-1908 and Similar Events in Light of the New Explosive Cosmogony of Minor Bodies

Update 30/3/09: Edward Drobyshevski writes to correct an error in this post. He says:

“In http://www.technologyreview.com/blog/arxiv/23234/ you write: “He says the comet would have had a high hydrogen peroxide content and this would have dissociated explosively as it heated up to produce oxygen and water, breaking the comet apart.” It is incorrect statement. I never said about the hydrogen peroxide. I am saying on the solid-state solution (=clathrate) of hydrogen and oxygen (=products of the volumetric electrolytical decomposition of the dirty ice) in the low-temperature ice under the great pressure and continuous solid-state thermal convection in the parent planetary body (not in the comet nuclei themselves).Please, reread my articles and correct the text.”


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