Three times in the past century, sizable asteroids have streaked into Earth’s atmosphere and exploded in midair. Astronomers have thought that such intense, rapid-fire pummeling was an anomaly. But now a group of researchers says it may be the norm. Their new analysis, based on February’s window-shattering blast over Chelyabinsk, Russia, suggests—but falls short of proving—that the world should expect 10 times as many atmospheric blasts as astronomers have been expecting.
There is no doubt that Chelyabinsk’s blast was well observed. There were the 400 video recordings of the smoke trail and blinding detonation, many made by cameras mounted in Russians’ cars to foil fraudsters faking traffic accidents. Seismographs gauged the ground shaking driven by the blast’s shock wave. And instruments monitoring the atmosphere’s ultra-low-frequency “infrasound,” many installed for the enforcement of the nuclear test-ban treaty, picked up the blast’s far-traveling sound.
Then there were the “U.S. government sensors,” as the paper reporting the re-analysis in tomorrow’s issue of Nature delicately puts it. As a condition of using the data from these sensors, the 33 authors—led by planetary scientist Peter Brown of the University of Western Ontario in London, Canada—can’t say any more than that about their data source. But it is common knowledge that the U.S. government flies light-sensing instruments on classified satellites to catch the flash of clandestine atmospheric nuclear tests.
Using all four types of observations, the group sized up the Chelyabinsk airburst at an energy equivalent to about 500 kilotons of TNT, about 23 times the energy of the nuclear bomb dropped on Nagasaki. That energy rating could then be used to calibrate the past 20 years of infrasound and U.S. government sensors, a record twice as long as analyzed in the past. By looking at the 60 1-meter to about 20-meter asteroids that have exploded in the atmosphere, the team found that the Chelyabinsk airburst and the 3- to 15-megaton Tunguska airburst of 1908—which leveled 2000 square kilometers of Siberian forest—weren’t outliers at all.
The authors conclude that the number of big airbursts “may be an order of magnitude higher” than astronomers would have predicted, they write. That would boost the frequency of Tunguska-size blasts from once in several thousand years to once in a few hundred years, Brown says. But he is not wedded to the order-of-magnitude figure. Given the small number of larger impactors observed, “I could easily see a factor of three [uncertainty] each way,” he says.
The low end of Brown’s range of airburst frequency is still higher than the astronomers’ estimate, but there may be a middle ground. He considers the lower estimate of impact frequency from telescopic surveys made by NASA consultant Alan Harris of La Cañada Flintridge, California, to be the best possible at this point. And Harris writes in an e-mail that his estimate is uncertain by a factor of two to three. So the high end of his estimated range overlaps with the low end of Brown’s range. Harris tends to think the overlap at an impact rate two to three times as high as previously estimated “is probably right.”
Researchers should soon know whether they can agree on a modest boost to the meteoric blast threat. More and better ground-based telescopes are being pressed into service in order to find a suitable target in the 5- to 10-meter size range for NASA’s Asteroid Redirect Mission. “This will be resolved in the next few years,” Brown says.
*Correction, 7 November, 11 a.m.: The original version of this story stated that the Tunguska meteoric explosion occurred in 1907. It actually occurred in 1908.