There are scads of building-size, potentially hazardous asteroids lurking in Earth’s immediate neighborhood, according to studies that examined the airburst of a 25 million-pound asteroid this year over the Russian city of Chelyabinsk.
When the asteroid exploded over Chelyabinsk in February, shattering windows for miles and injuring more than 1,200 people, experts said it was a rare event — of a magnitude that might occur once every 100 to 200 years, on average.
But a team of scientists is suggesting that the Earth is vulnerable to many more Chelyabinsk-size space rocks than was previously thought. In research published Wednesday in the journal Nature, scientists estimate such strikes could occur as often as every decade or two.
The prospect “really makes a lot of people uncomfortable,” said Peter Brown, a professor of physics and astronomy at the University of Western Ontario and an author of two studies in Nature. A third paper by other scientists describing the Chelyabinsk explosion was published online this week by the journal Science.
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Meteors about the size of the one that burst over Chelyabinsk — 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 the three studies.
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.
The findings are helping to elevate the topic of planetary defense — identifying dangerous asteroids and deflecting them if necessary — from Hollywood fantasy to real-world concern.
A U.N. committee has been studying the issue, and next month the General Assembly is expected to adopt two of its recommendations: establishing an International Asteroid Warning Network for countries to share information; and calling on the world’s space agencies to set up an advisory group to explore technologies for deflecting an asteroid.
Sky surveys have already detected about 95 percent of the big near-Earth asteroids, those that are at least 1 kilometer wide, or 0.6 miles, and none are in danger of hitting Earth anytime soon.
But those are not the only ones to worry about.
“One kilometer is more than just dangerous,” said Edward Lu, a former NASA space-shuttle astronaut who heads the B612 Foundation, a private effort to launch a space telescope that could find smaller asteroids. “One kilometer is end-of-human-civilization kind of dangerous.”
The Chelyabinsk asteroid was about 65 feet wide. Undetected by astronomers, the rock came out of the glare of the sun and hit the atmosphere at 43,000 mph.
As it descended through the atmosphere, it broke into fragments, creating a series of explosions with the combined energy of about 500,000 tons of TNT, making it more than 30 times more powerful than the atomic bomb that destroyed Hiroshima, Japan, in 1945, although the energy in this case was spread out over a much broader area.
The shock wave blew out windows in nearly half the buildings in Chelyabinsk. It knocked people off their feet; dozens were sunburned by the blinding flash, which at its peak was 30 times brighter than the sun.
Most of the people who were injured were hit by broken and flying glass; no one was killed.
One chunk the size of love seat landed in frozen Lake Chebarkul, leaving a circular hole, as if shot with a bullet from space. Thousands of smaller pieces have also been recovered.
The new information on Chelyabinsk does not suggest the sky is falling, but it shifts the risk profile of asteroids, making Chelyabinsk-size events look more probable.
Brown re-examined decades of data compiled by scientific and military sensors. The scientific orthodoxy said a Chelyabinsk-size event ought to happen every 140 years or so, but Brown saw several such events in the historical record.
“Any one of these taken separately I think you can dismiss as a one-off. But now when we look at it as a whole, over a hundred years, we see these large impactors more frequently than we would expect,” said Brown.
The paper in Science hypothesized that the Chelyabinsk asteroid is a piece of “rubble” from a larger body that had been broken apart by tidal forces from an earlier near-Earth encounter. “The rest of that rubble could still be part of the near-Earth object population,” the authors wrote.
A proposed B612 telescope, to be called Sentinel, is designed to find asteroids about 450 feet wide, although it will also find many that are smaller. Lu, the former shuttle astronaut, said the mission would cost $450 million: $250 million to build the spacecraft and $200 million to operate it for a decade.
Many of the Chelyabinsk-size asteroids would elude detection by Sentinel. Still, the residents of Chelyabinsk would have benefited from a warning Feb. 15 to stay away from the window.
Material from The Associated Press is included in this report.