Traci Watson, Special for USA TODAY

Space rocks big enough to destroy a city hit the Earth much more often than thought, according to an estimate by a private group devoted to preventing disaster from such orbital killers.

It took a space rock the size of San Francisco to finish off the dinosaurs 65 million years ago, but a decent-sized metropolis could be reduced to smoldering ruins by a boulder that could fit inside a soccer field. The strike rate for such large space rocks, properly known as asteroids, has been estimated at once every 3,000 years, but the B612 Foundation, a planetary defense group, says the true figure could be as high as once a century. Outside scientists say that frequency is plausible but could well be too high.

"There are people who say, 'Oh, once every million years we have something we have to worry about.' That couldn't be more wrong," says physicist and former space shuttle astronaut Ed Lu, chief executive officer of the B612 Foundation. "Eventually you're going to get hit, because it's just a matter of time."

B612's once-a-century estimate comes in part from data collected by a worldwide network of sensors designed to detect nuclear explosions. The sensor network, which became a global system only within the past decade, picks up the ultra-deep sound waves that circle the Earth after a nuclear bomb is detonated in the atmosphere or just under the Earth's surface or when an asteroid explodes in midair. The data were made available to the foundation by Peter Brown of the University of Western Ontario, who will present the numbers at a scientific conference this summer.

Since 2000, the sensors have detected 26 asteroid blasts equivalent to at least 1,000 tons of TNT. In four of those incidents, the space rock's explosion unleashed more energy than the atomic bomb that leveled Hiroshima. The self-destruction of an asteroid roughly as big as a house in 2009 just off the Indonesian coast was more powerful than three Hiroshima bombs. An even bigger asteroid explosion over Chelyabinsk, Russia, in 2013 injured more than 1,000 people, blew out windows and created a fireball bright enough to give some witnesses a sunburn.

None of the 26 asteroids in Brown's data was big enough to destroy a city, because of their size and composition and because all exploded high in the atmosphere. But Lu says this database of harmless asteroids can be extrapolated to shed light on the frequency of their fearsome cousins. The results suggest that a city-killer strikes once a century, though Lu says he wouldn't be surprised if the true rate were actually less worrisome, perhaps once every 150 or 200 years or less frequent still.

Lu's group is raising money to build an orbiting telescope to spot potentially destructive asteroids, including those big enough to wipe out a city, decades before they strike the Earth. Numerous telescopes around the world survey the sky for asteroids, and more instruments are coming online. B612 says its orbiting craft would be far more efficient at spotting space rocks that are relatively small but deadly.

Brown agrees that the true hit rate for asteroids of that size is probably more frequent than past estimates, but he's cautious about the B612 Foundation's once-a-century number.

"It could be that high," Brown says, but once every 100 years "is probably very near the upper range of numbers." The true number is probably closer to once in 500 years, or even once in 1,000 years, he says. In a 2013 study, Brown and his colleagues found that impacts by asteroids ranging from bus-sized to department-store-sized could be 10 times more common than thought, but a definitive number is impossible, given the skimpy database of city-killers. Scientists have details about exactly one recent asteroid strike large enough to wipe out an urban area: the Tunguska event, when an asteroid perhaps 180 feet wide exploded over the wilderness of Siberia in 1908, leveling 800 square miles of forest.

"It is quite possible that Tunguska is a once-per-century event, but it is also possible that it's a once-per-three-centuries event," says Brown's co-author Mark Boslough of Sandia National Laboratories via e-mail, who notes that other experts think the hit rate is much lower. "We need more surveys … to resolve this."

Source and special thanks: USA Today

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