The meteor that exploded over Russia on Friday set off the largest explosion of its kind in more than a century, scientists say. It caused shock waves that shattered windows over 20 hectares and injured 1200 residents of the city of Chelyabinsk.
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It hit on the same day astronomers were watching another larger rock miss Earth by just 28,000 kilometres, and scientists scrambled to understand the two rare cosmic events.
Some initially speculated that Earth was passing through a swarm of asteroids but the Russian meteor came from the other direction.
''There is no relation there,'' said Paul Chodas, a scientist with NASA's Near Earth Object Program. ''It seems like we're in a cosmic shooting gallery here. There were two very rare events happening on the same day. Pure coincidence.''
Based on preliminary calculations, the meteor that shook Chelyabinsk weighed about 7000 tonnes and was about 15 metres in diameter when it entered the atmosphere at 60,000km/h about 9.20am local time, a professor of physics at the University of Western Ontario, Peter Brown, said.
Professor Brown based his calculations on low-frequency sound waves that travelled as far away as Alaska. A worldwide network of such sensors listens for these vibrations, too low to be heard by humans, to verify nuclear tests.
Russian scientists estimated a much smaller size, just three metres across and weighing 10 tonnes.
The meteor streaked across the sky for about half a minute, unleashing thunderous sonic booms. It exploded at 20 to 25 kilometres above the surface and released the equivalent of 300,000 tonnes of TNT of energy, Professor Brown said, although that initial estimate could be revised higher, perhaps to 500,000 tonnes of TNT energy.
The atomic bomb that destroyed Hiroshima during World War II was the equivalent of 15,000 tonnes of TNT.
''This is the largest recorded event since the 1908 Tunguska event,'' Mr Chodas said. On June 30, 1908, the explosion of a meteor, believed to be an asteroid, flattened millions of trees over 200,000 hectares in a remote, largely uninhabited area of central Siberia about 2000 kilometres away from Friday's strike.
At a NASA news conference on Friday the leader of the Meteoroid Environment Office at the Marshall Space Flight Centre in Huntsville, Alabama, Bill Cooke, said the meteor was impossible to detect because it approached from the day side. ''And as you know, telescopes can't see anything during the day,'' he said.
In Boulder, Colorado, the Southwest Research Institute's department of space studies scientist Clark Chapman said this was the first time a crashing meteor was known to have injured a large number of people.
Giant impacts have changed the course of life on Earth, notably 65 million years ago when an object several kilometres wide slammed off the coast of Mexico and killed off the dinosaurs.
The New York Times
