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 about 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 figured a meteor causing such an airburst was a one-in-150-year 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 they are actually hitting Earth.
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, said the space agency is reassessing what size space rocks to look for and how often they are likely to hit.
The U.S. government gained a new sense of urgency after Chelyabinsk, quietly holding a disaster drill earlier this year in Washington that was meant to simulate what would happen if a slightly bigger space rock threatened the East Coast.
After the drill, NASA and Federal Emergency Management Agency officials said they should look at the need for evacuations, figure out ways of keeping the public informed without scaring them, and handle meteor threats in a way comparable to how they deal with hurricanes bearing down on the coast.
“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.”
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 span.
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.
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 space rocks and a 1.2-mile-wide one that had escaped their notice until this month.
The three objects won’t hit Earth, but their discovery raises the question of why they weren’t seen until now.
The last time a 12-mile-wide rock had been discovered was about 30 years ago, and two popped into scientists’ view just now, NASA asteroid scientist Donald Yeomans said. He said NASA had thought it had already seen 95 percent of the large space rocks that come near Earth.