Scientists studying the dramatic Chelyabinsk meteor that screamed through Russian skies this year have both good and bad news to report. The good news: The February fireball's damage wasn't nearly as terrible as predicted. The bad news? Near-Earth object impacts could be about 10 times more common than we thought they were.
The results, presented in one of three studies published in the journals Nature and Science, show that scientists on the ground may have to reevaluate how we predict both the frequency of such impacts and our understanding of how they behave when they enter our atmospheres.
Scientists flocked to Chelyabinsk in the meteor's wake. The roughly 19-meter-wide space rock that shattered in mid-air and sent out a shock wave that broke glass windows and injured more than 1,500. Hundreds of cameras captured the event, making it an ideal chance for some hands-on research, scientists said.
"Clearly it was a once-in-a-lifetime opportunity," said Peter Brown, a planetary scientist at the University of Western Ontario who led one of the two studies in Nature.
The scientists used plentiful footage from 400 video cameras (typically dashboard cameras sitting in cars) and a host of data sources, from infrasound to seismic, to characterize the meteor's impact. They found that it exploded with the energy of 500 kilotons of TNT – roughly equivalent to a modern nuclear bomb. For an observer standing right beneath the blast, it would have appeared 30 times brighter than the sun when it disintegrated.
Still, Chelyabinsk didn't do as much damage as would be predicted by a blast that strong. They think it's in part because a space rock that breaks up in our atmosphere and releases energy does so while it's moving, which distributes energy differently than a point-source like a nuclear bomb detonation, Brown said.
"We need to refine those estimates," he said, and produce models that really capture the complexity of such impacts.
The scientists also wanted to see how accurate our current telescope-based asteroid trackers were. They used decades of data from infrasound instruments and U.S. government sensors to pull out records of shock waves created by asteroids hitting Earth's atmosphere. Then they compared it to the telescopic data.
"We should see something like Chelyabinsk every 30 to 40 years rather than every 120 to 140, so a factor of three or four more of these impacts than the telescopic data suggest," Brown said.