Earth is far more vulnerable than previously thought to asteroids like the one that exploded over the Russian city of Chelyabinsk in February, injuring over 1,000 people and shattering windows for miles, say scientists in new research
While experts previously said a strike of the Chelyabinsk  magnitude might occur only once every one or two centuries, research published in the journal Nature estimates they could happen every 10 to 20 years, The New York Times reported.
That “really makes a lot of people uncomfortable,” Peter G. Brown, a professor of physics and astronomy at the University of Western Ontario and an author of the studies in Nature, told The Times.
The Chelyabinsk asteroid  was only 60 feet wide, The Times reported, going 40,000 miles per hour and releasing the equivalent energy of 500,000 tons of TNT upon explosion. An asteroid perhaps two or three times the diameter of that one blew up over Siberia in 1908 and is thought to have released energy equal to 5 to 10 million tons of TNT, leveling millions of trees.
A United Nations committee has been studying the issue. The General Assembly is set to adopt recommendations to establish an International Asteroid Warning Network to share information and call on space agencies to establish an advisory group to look into asteroid-deflecting technologies.
Sky surveys have located 95 percent of asteroids near Earth that are at least a kilometer wide, and none look to hit the planet anytime soon.
“One kilometer is more than just dangerous,” Edward T. Lu, a former NASA astronaut who heads the B612 Foundation, a private effort to launch a space telescope that could find smaller but still dangerous asteroids, told The Times. “One kilometer is end-of-human-civilization kind of dangerous.”
But smaller ones that could hit Earth every decade or so are still quite deadly.
The telescope Dr. Lu wants to launch would detect asteroids about 450 feet wide — not global doomsday material, but enough to cause mass casualties and wreak catastrophic economic damage.
“You’re not going to wipe out humanity,” he told The Times, “but if you get unlucky, you could kill 50 million people or you could collapse the world economy for a century, two centuries.”
He added that astronomers have only found about 10 to 20 percent of near-Earth asteroids that size, and the telescope, called Sentinel, would also locate smaller ones the size of the 1908 Siberia asteroid, only 0.5 percent of which have been found.
“What we’ve been talking about are the ones that would only destroy a major metropolitan area — all of New York City and the surrounding area,” Dr. Lu told The Times.
For the research into potential asteroid hits, Dr. Brown and his colleagues looked into what actually has hit the Earth, since telescope surveys have found so few of the smaller ones. According to The Times, they examined Air Force data from the 1960s and 1970s and later data from sensors designed to verify a ban on aboveground nuclear testing. They concluded that larger asteroids hit the Earth more frequently than estimates based on sky surveys would suggest, meaning that either the Earth has been unlucky lately, or estimates for Chelyabinsk-size asteroids are too low.
“Any one of them individually I think you could dismiss,” Dr. Brown told the Times, “but when you take it all together, I think the preponderance of the evidence is there is a much higher number of these tens-of-meters-size objects.”
Astronomers from the University of Hawaii are setting up telescopes to look for quick-moving spots of light that could come from asteroids, the newspaper said. The project, funded by a $5 million grant from NASA and set to go into operation in 2015, wouldn’t give enough time to deflect an asteroid but would provide ample warning to evacuate.