Large meteors more likely to hit Earth than previous thought
Studies warn of more dangerous fireballs landing more frequently
Scientists studying the terrifying meteor that exploded without warning over a Russian city last winter say that it shows the threat of space rocks smashing into earth is bigger than they thought.
Meteors about the size of the one that streaked through the sky at 67,600km/h 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 on Wednesday in the journals Nature and Science.
That means about 20 million space rocks the size of the Chelyabinsk one could be zipping around the solar system, instead of three million, Nasa scientist Paul Chodas said.
Until Chelyabinsk, Nasa had looked only for space rocks about 30 metres wide and bigger, figuring there was little danger in the ones below that.
The Chelyabinsk meteor was only 19 metres 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 in icy Russia. More than 1,600 people 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 the 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, the 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.
Nasa this autumn reactivated a dormant orbiting telescope called Wise specifically to hunt for asteroids, Johnson said. And the agency is also expanding ground-based sky searches that might give a few extra days' notice of smaller meteors like the Russian one.
At the same time, Nasa and the Federal Emergency Management Agency are looking into the need for evacuations in the case of an immediate threat and how to keep the public informed without scaring people.
The two agencies held a disaster drill last spring in Washington meant to simulate what would happen if a space rock slightly bigger than the Chelyabinsk one threatened the US East Coast.
During the drill, when it looked as if the meteor would hit just outside the US capital, experts predicted 78,000 people could die.