The meteor that exploded over the Siberian city of Chelyabinsk in February released more than 30 times the explosive energy of the Hiroshima bomb, a study has found. 
 
 Scientists calculated that the blast caused by the disintegration of the space rock as it ploughed through the atmosphere at 19 kilometres per second was equivalent to between 500,000 and 600,000 tons of TNT – compared with the 16,000-ton explosion at Hiroshima in 1945. 
 
 People directly under the flight of the meteor were knocked off their feet and many others suffered sunburn or eye damage as they looked at the intense fireball. 
 
 It was the biggest meteor blast since the devastating explosion over Tunguska, also in Siberia, in 1908, which flattened forests for hundreds of miles around and was reckoned to have released energy equivalent to between 5 million and 15 million tons of TNT. 
 
 The Chelyabinsk meteorite – the part of the meteor that fell to Earth – belongs to the most common class of space rock, known as ordinary chondrites. It is these that are most likely to cause a meteorite catastrophe, said Professor Qing-Zhu Yin of the University of California, Davis. 
 
 “If humanity does not want to go the way of the dinosaurs, we need to study an event like this in detail. Chelyabinsk serves as a unique calibration point for high-energy meteorite impact events for our future studies,” Professor Yin said. 
 
 Two independent studies, published in the journals Nature and Science, agree closely on the strength of the explosive force as the meteor disintegrated south of Chelyabinsk, punching an eight-metre wide hole in frozen surface of a lake. 
 
 Calculations suggest that the meteor broke into smaller pieces at altitudes of 45km and then 30km, preventing more serious injuries and damage on the ground. Even so, windows were shattered for miles around, the scientists said. 
 
 “Our goal was to understand all circumstances that resulted in the damaging shock wave,” said Peter Jenniskens, of Nasa, one of dozens of researchers who took part in the studies. 
 
 Olga Popova of the Russian Academy of Sciences in Moscow, who led one of the studies, said: “Our [meteor] entry modelling showed that the impact was caused by a 20-metre chunk of rock that fragmented at 30km.” At that point the space rock glowed brighter than the Sun. 
 
 The shockwave damaged buildings up to 50 miles either side of the meteor’s east-west trajectory. The largest remains of the meteorite were eventually recovered from the bed of the lake. 
 
 Tests showed that it was about 4,567m years old – about the same age as the Earth. ||||| Media playback is unsupported on your device Media caption Archive: Eyewitnesses and CCTV cameras captured the moment the meteor flew across the sky in Russia in February 
 
 The threat of another asteroid strike like the one that hit Russia earlier this year is much higher than was previously thought, a study suggests. 
 
 Researchers have found that space rocks of a similar size to the one that exploded over Chelyabinsk are hurtling into the Earth's atmosphere with surprising frequency. 
 
 Scientists say early warning systems need to be put in place. 
 
 The study is published in the journal Nature. 
 
 Lead author Professor Peter Brown, from the University of Western Ontario, Canada, told the BBC World Service's Science in Action programme: "Having some sort of system that scans the sky almost continuously and looks for these objects just before they hit the Earth, that probably is something worth doing. 
 
 "In the case of Chelyabinsk, a few days' to a week's warning would have been valuable." 
 
 Fireball 
 
 The asteroid that exploded over Russia on 15 February this year was estimated to be about 19m-wide. 
 
 There are literally millions of objects in the tens of metres of size range that we suspect are near Earth asteroids Prof Peter Brown, University of Western Ontario 
 
 It hit the atmosphere with energy estimated to be equivalent to 500,000 tonnes of TNT, sending a shockwave twice around the globe. It caused widespread damage and injured more than 1,000 people. 
 
 Now though, scientists say there could be many more space rocks like this one on a collision course with the Earth. 
 
 An international team looked at the last 20 years of data collected from sensors used by the US government and infrasound sensors positioned around the globe. 
 
 These detectors are in place to detect the threat of nuclear weapons, but they can also capture the blasts caused by asteroid impacts. 
 
 The researchers found that during this time about 60 asteroids up to 20m in size had smashed into the Earth's atmosphere: far more than was previously thought 
 
 Most went undetected because they exploded over the ocean or in very remote areas. 
 
 Prof Brown explained: "We were able to capture the occurrence rate you would expect of things like Chelyabinsk and smaller impacts. When you compare that to the numbers you get from telescopic [observations], our numbers are several times higher." 
 
 Media playback is unsupported on your device Media caption Archive: Divers recently recovered a large piece of the Chelyabinsk meteorite from a lake 
 
 This suggests that the risk from asteroids of this scale has been underestimated. The team estimates that the strike rate of asteroids that are tens of metres in size is between two and 10 times higher than was previously thought. 
 
 "Something like Chelyabinsk, you would only expect every 150 years on the basis of the telescopic information. But when you look at our data and extrapolate from that, we see that these things seem to be happening every 30 years or so," said Prof Brown. 
 
 An event such as the Tunguska impact in 1908, where an asteroid flattened thousands of square kilometres of forest in Siberia, would probably happen every few hundred years rather than every few thousand years, he added. 
 
 Image caption Many people in Russia caught the Chelyabinsk meteor on camera 
 
 Prof Brown said more effort should be made to create early warning systems. 
 
 "There are literally millions of objects in the tens-of-metres-of-size range that we suspect are near Earth asteroids, that can get close to the Earth," he said. 
 
 "We have only discovered over 1,000 of these. There are many more of these to find, but it would be very expensive to find all of these and it probably wouldn't make a lot of sense because the atmosphere largely stops them. 
 
 "But what might make sense are systems that find something a few days or weeks before they hit... to tell you where on the Earth and when they will hit. That would allow some warning to be given to the civil defence authorities." 
 
 In another study, also published in Nature, scientists believe they have traced the asteroid that the Chelyabinsk meteor splintered off from. 
 
 They think it is a fragment of a 2km-wide rock called asteroid 86039. 
 
 Professor Jiri Borovicka, from the Academy of Sciences of the Czech Republic, said the orbits were "conspicuously similar", although the team could not prove "the common origin with absolute certainty". 
 
 Follow Rebecca on Twitter ||||| Scientists studying the terrifying meteor that exploded without warning over a Russian city last winter say the threat of space rocks smashing into Earth is bigger than they thought. 
 
 Maps show how scientists will narrow the field of impact in the weeks approaching the time of impact.; 2c x 5 inches; 96.3 mm x 127 mm; (Associated Press) 
 
 FILE - In this Feb. 15, 2013 file photo provided by Chelyabinsk.ru, shows a meteorite contrail over the Ural Mountains' city of Chelyabinsk, about 930 miles east of Moscow, Russia. After a surprise meteor... (Associated Press) 
 
 This photo provided by The Field Museum in Chicago, taken April 9, 2013, shows pieces of a meteor that exploded over Russia's Ural Mountains in February after they were catalogued on their arrival at... (Associated Press) 
 
 In this frame grab made from dashboard camera vide shows a meteor streaking through the sky over Chelyabinsk, about 930 miles east of Moscow, Friday, Feb. 15, 2013. After a surprise meteor hit Earth at... (Associated Press) 
 
 Meteors about the size of the one that streaked through the sky at 42,000 mph and burst over Chelyabinsk in February _ and ones even larger and more dangerous _ are probably four, five or even seven times more likely to hit the planet than scientists believed before the fireball, according to three studies published Wednesday in the journals Nature and Science. 
 
 That means about 20 million space rocks the size of the Chelyabinsk one may be zipping around the solar system, instead of 3 million, NASA scientist Paul Chodas said at a news conference. 
 
 Until Chelyabinsk, NASA had looked only for space rocks about 100 feet wide and bigger, figuring there was little danger below that. 
 
 This meteor was only 62 feet across but burst with the force of 40 Hiroshima-type atom bombs, scientists say. Its shock wave shattered thousands of windows, and its flash temporarily blinded 70 people and caused dozens of skin-peeling sunburns just after dawn in icy Russia. More than 1,600 people in all were injured. 
 
 Up until then, scientists had figured a meteor causing an airburst like that was a once-in-150-years event, based on how many space rocks have been identified in orbit. But one of the studies now says it is likely to happen once every 30 years or so, based on how often these things are actually hitting. 
 
 By readjusting how often these rocks strike and how damaging even small ones can be, "those two things together can increase the risk by an order of magnitude," said Mark Boslough, a Sandia National Lab physicist, co-author of one of the studies. 
 
 Lindley Johnson, manager of NASA's Near Earth Object program, which scans the heavens for dangerous objects, said the space agency is reassessing what size rocks to look for and how often they are likely to hit. 
 
 In addition, NASA this fall reactivated a dormant orbiting telescope called WISE specifically to hunt for asteroids, Johnson said. And the agency is expanding ground-based sky searches. 
 
 At the same time, NASA and the Federal Emergency Management Agency are looking into the need for evacuations in the case of an asteroid headed for Earth and how to keep the public informed without scaring people. 
 
 Those issues came up after the two agencies quietly held a disaster drill last spring in Washington that was meant to simulate what would happen if a space rock slightly bigger than the Chelyabinsk one threatened the East Coast. 
 
 During the drill, when it looked as if the meteor would hit just outside the nation's capital, experts predicted 78,000 people could die. But when the mock meteor ended up in the ocean, the fake damage featured a 49-foot tsunami and shortages of supplies along the East Coast, according to an after-action report obtained by The Associated Press. 
 
 The exercise and the studies show there's a risk from smaller space rocks that strike before they are detected _ not just from the giant, long-seen-in-advance ones like in the movie "Armageddon," said Bill Ailor, a space debris expert at the Aerospace Corporation who helped coordinate the drill. 
 
 "The biggest hazard from asteroids right now is the city-busting airbursts, not the civilization-busting impacts from 1-kilometer-diameter objects that has so far been the target of most astronomical surveys," Purdue University astronomer Jay Melosh, who wasn't part of the studies, wrote in an email. 
 
 "Old-fashioned civil defense, not Bruce Willis and his atom bombs, might be the best insurance against hazards of this kind." 
 
 Chodas said the Chelyabinsk rock surprised astronomers because it was coming from the direction of the sun and was not detectable. Telescopes can see some space rocks as small as 3 feet wide, but some are simply too dark to spot, he said. 
 
 Scientists said a 1908 giant blast over Siberia, a 1963 airborne explosion off the coast of South Africa, and others were of the type that is supposed to happen less than once a century, or in the case of Siberia, once every 8,000 years, yet they all occurred in a 105-year timespan. 
 
 Because more than two-thirds of Earth is covered with water and other vast expanses are uninhabited deserts and ice, other past fireballs could have gone unnoticed. 
 
 Just this week, NASA got a wake-up call on those bigger space rocks that astronomers thought they had a handle on, discovering two 12-mile-wide asteroids and a 1.2-mile-wide one that had escaped their notice until this month. However, NASA said the three objects won't hit Earth. 
 
 Asteroids are space rocks that circle the sun as leftovers of failed attempts to form planets billions of years ago. When asteroids enter Earth's atmosphere, they become meteors. (When they hit the ground, they are called meteorites.) 
 
 The studies said the Chelyabinsk meteor probably split off from a much bigger space rock. 
 
 What happened in the Russian city of 1 million people is altering how astronomers look at a space rocks. With first-of-its-kind video, photos, satellite imagery and the broken-up rock, scientists have been able to piece together the best picture yet of what happens when an asteroid careens into Earth's atmosphere. It's not pretty. 
 
 "I certainly never expected to see something of this scale or this magnitude," said University of Western Ontario physicist Peter Brown, lead author of one study. "It's certainly scary." 
 
 Scientists said the unusually shallow entry of the space rock spread out its powerful explosion, limiting its worst damage but making a wider area feel the effects. When it burst it released 500 kilotons of energy, scientists calculated. 
 
 "We were lucky. This could have easily gone the other way. It was really dangerous," said NASA meteor astronomer Peter Jenniskens, co-author of one of the papers. "This was clearly extraordinary. Just stunning." 
 
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 Online: 
 
 NASA's Near Earth Object Program: http://neo.jpl.nasa.gov 
 
 Nature: http://www.nature.com/nature 
 
 The journal Science: http://www.sciencemag.org 
 
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 Seth Borenstein can be followed at http://twitter.com/borenbears
– A meteor strike like the one in Chelyabinsk, Russia, earlier this year might seem like a once in a lifetime event, but a new study in Nature says that isn't the case. Using data from sensors around the world, researchers found that big asteroids have hit Earth's atmosphere with "surprising frequency," the BBC reports—it's just that most explode over oceans or remote areas. "Something like Chelyabinsk, you would only expect every 150 years on the basis of the telescopic information. But when you look at our data and extrapolate from that, we see that these things seem to be happening every 30 years or so," says the lead author. The frequency of such hits might be up to seven times greater than previously thought, reports AP. Ideally, scientists would have had a few days or weeks' notice of the strike, which is why "having some sort of system that scans the sky almost continuously and looks for these objects just before they hit the Earth, that probably is something worth doing." says the researcher. Meanwhile, a separate study in Nature has found the source of Russia's meteor strike. Researchers think the rock that hit Chelyabinsk broke off from a mile-wide asteroid known as 86039—resulting in an impact that created 30 times the explosive power of the Hiroshima bomb, the Independent reports. "If humanity does not want to go the way of the dinosaurs, we need to study an event like this in detail," says one professor.