When an asteroid exploded over the Russian city of Chelyabinsk in February, shattering windows for miles and injuring well over 1,000 people, experts said it was a rare event — of a magnitude that might occur only once every 100 to 200 years, on average. But now a team of scientists is suggesting that the Earth is vulnerable to many more Chelyabinsk-size space rocks than was previously thought. In research being published on Wednesday by the journal Nature, they estimate that such strikes could occur as often as every decade or two. The prospect “really makes a lot of people uncomfortable,” said Peter G Brown, a professor of physics and astronomy at the University of Western Ontario and an author of the two studies in Nature. A third paper by other scientists describing the Chelyabinsk explosion was published online this week by the journal Science. 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 United Nations committee has been studying the issue for some time, 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 spotted about 95 per cent of the big near-Earth asteroids, those that are at least one 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 T. 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-ofhuman- civilization kind of dangerous.” The Chelyabinsk asteroid was just 60 feet wide. Speeding around 40,000 miles per hour, it released energy equal to 500,000 tons of TNT. A larger asteroid, perhaps two or three times the diameter of the Chelyabinsk one, exploded over Siberia in 1908 and is estimated to have released energy equivalent to 5 million to 15 million tons of TNT, flattening millions of trees.

The proposed B612 telescope, to be called Sentinel, is intended to find asteroids about 450 feet wide, although it will also find many that are smaller. Dr Lu said the mission would cost $450 million — $250 million to build the spacecraft and $200 million to operate it for a decade. A 450-foot-wide asteroid, Dr Lu said, would be equivalent to 150 million tons of TNT. “You’re not going to wipe out humanity,” he said, “but if you get unlucky, you could kill 50 million people or you could collapse the world economy for a century, two centuries.” Dr Lu said astronomers had found only 10 to 20 per cent of the near-Earth asteroids of that size. Sentinel would also spot many smaller ones that could still be devastating. “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 said. He said only about 0.5 per cent of these smaller asteroids, roughly the size of the 1908 one, have been found. Because telescope surveys have counted so few of the small asteroids, Dr Brown and his colleagues instead investigated what has actually hit the Earth. In one of the articles in Nature, they examined United States Air Force data from the 1960s and 1970s and later data from sensors verifying a ban on aboveground nuclear weapons testing.

The recordings captured the lowfrequency atmospheric rumblings generated by about 60 asteroid explosions. Most came from small asteroids, but their data suggested that the somewhat larger ones hit more frequently than would be expected based on the estimates from sky surveys. That could mean the Earth has been unlucky recently, or that the estimates on the number of Chelyabinsk-size asteroids are too low. “Any one of them individually I think you could dismiss,” Dr Brown said, “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.” Dr Lu said that was one more reason to launch an asteroid-finding telescope. “There are hints the rate is higher than we think, but we don’t really know yet, and I think we should find out,” he said. “When you find out how many there are, you also find out where the individual ones are. Everything you discover you can either rule out as going to hit us or you say, ‘Hey, we ought to look at this one more carefully.'” Many of the Chelyabinsk-size asteroids would elude detection by Sentinel. Still, the residents of Chelyabinsk would have benefited from a warning on the morning of Feb. 15 to stay away from the window. With a $5 million grant from NASA, University of Hawaii astronomers are setting up telescopes to scan the sky for quickmoving spots of light that could be oncoming asteroids. There would be no opportunity to deflect the asteroid that would hit in days or weeks, but it would give time to warn, and to evacuate. That system is scheduled to go into operation in 2015.