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. It also looking into how the government should react if an asteroid is headed for Earth.
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 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.
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 that are 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 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."