One Year Later: Russian Meteor is a Science Boon

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"The asteroid broke into small pieces between the altitudes of around 45 and 30 kilometers (28 and 18 miles), preventing more serious damage on the ground," scientists in the Czech Republic and Canada reported last November.

The trajectory, they added, suggested it was once part of a massive two-kilometer (1.2-mile) rock called 86039, a nasty "geocruiser" first spotted in 1999 that regularly comes close to Earth's orbit.

Meteorite hunters have gone into overdrive to pick up fragments, including a mighty half-tonne chunk recovered from a lake near Chelyabinsk.

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Chemical analysis revealed the meteorite was a so-called LL chondrite, which constitute the minority of asteroids, but with a high content of cobalt.

Grokhovsky said the fragments had intriguing differences. Some are blackened on top, others all the way through and others are half-and-half.

"Maybe it is radiation from space, the impact, the melting," he said.

Investigators will meet in Houston, Texas from March 17-21 at the annual Lunar and Planetary Science Conference, which is devoting a special session to the Chelyabinsk event.

The last time that a big rock is known to have collided with Earth was on June 30, 1908, when an object up to 70 meters (227 feet) across exploded over Tunguska, Siberia, flattening 80 million trees over 2,000 square kilometers (800 square miles).

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The Tunguska and Chelyabinsk asteroids belong in the lower ranks of the medium category of asteroid risk, say astrophysicists.

Asteroids in the middle category can inflict local and, at the top end of the scale, regional damage.

But they are too small to belong to the class of civilization-wipers more than a kilometer (half a mile) across, capable of inflicting a mass extinction.

This is likely what happened 65 million years ago, when the long reign of the dinosaurs was ended by climate change thought to have been brought on by an impact in the Yucatan peninsula in modern-day Mexico.

The unsettling news, though, is that the risk from Tunguska-sized objects may have to be reviewed in the light of what happened at Chelyabinsk.

"Rather than running into Earth at an interval of around 4,500 years, as conventional wisdom has it, Tunguska-sized objects appear to be coming in to hit the Earth maybe every several hundred years," said Bailey.

"That's quite a lot more often."

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