Chunks of rock fall from space all the time, some of them big enough to do a fair amount of damage. Most of the time, however, nobody’s the wiser: the vast majority of Earth’s surface is ocean, desert or wilderness, so even a good-sized meteorite can fall without anyone noticing. And that’s assuming the incoming ordnance makes it through our atmosphere at all without burning up first.
That’s not how things played out last February 15, though, when a 60-ft.-wide (18 m) space boulder blazed through the skies above Chelyabinsk, Russia just after sunrise, then exploded in mid-air. The fiery meteor was so bright it briefly outshone the Sun, and even caused sunburns; its sonic boom, meanwhile, smashed thousands of windows and threw people to the ground. Nobody died in the incident, which lasted no more than 10 seconds or so, but about 1,500 people had to seek medical treatment for their injuries.
By falling in such a densely populated area, the Chelyabinsk meteor also became the most widely observed and thoroughly documented such event in history. And by drawing on camera footage, eyewitness accounts, the distribution of smashed windows, imagery from downward-looking satellites, seismic records and more, scientists have managed to piece together an extraordinarily detailed account of the rock’s brief passage. “This event has so many fantastic observations it’s incredible,” says Peter Jenniskens, of the NASA Ames Research Center and the SETI Institute, co-author of one of three papers that have just been published about those observations, one in Science and two in Nature.
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One statistic the dozens of scientists involved have nailed down is the enormous burst of energy released when the meteor exploded as it fought its way through the atmosphere at more than 40,000 m.p.h. (64,000 k/h)—a blast equivalent to about 500 kilotons of TNT, or more than 30 times the power of the A-bomb that leveled Hiroshima. The meteor, which originally weighed about 13,200 tons, had already begun breaking apart at an altitude of 27 miles (43 km) or so, and low-frequency sound recordings noted no fewer than 11 fragmentation events as it fell to 18 miles (29 km) above the ground.
At that point, it more or less exploded. “Two main fragments survived,” says Jenniskens, with the rest of the meteor vaporizing in a burst of heat that sent a mushroom cloud billowing upward. One of those chunks fragmented further, about 11 miles (18 km) above the ground; the remaining chunk ended up smashing through the ice covering Lake Chebarkul. “We were extremely lucky to find camera footage that actually filmed the chunk entering the lake,” says Jenniskens, who joined Russian colleagues in visiting 50 villages in the region to take measurements and record eyewitness testimony.
That half-ton fragment—a meteorite, which means it made it all the way to Earth, in contrast to a meteor, which burns up while still airborne—was finally located and hauled out of the water last month.
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The way the original meteor fragmented, meanwhile, suggested to researchers that it had been part of a larger asteroid that broke apart while still in space. And alterations in the chemical structure of the rock caused by cosmic-ray bombardment dated that breakup to about 1.2 million years ago. Based on its orbit, inferred from the detailed entry trajectory reconstructed from all the available evidence, that breakup probably didn’t happen when the parent body was in its original home in the asteroid belt between Mars and Jupiter. Instead, says Jenniskens, it would have happened during a close gravitational encounter with one of the inner planets.
“That suggests,” he says, “that the rest is still out there. People have looked, but haven’t found it yet.” Another asteroid fragment that screamed past Earth just a few hours after the Chelyabinsk event seemed like a likely candidate, but its trajectory suggests it was unrelated—which means the rogue rock is still at large.
That’s the bad news. The worse news is that there may be still more boulders of this size and destructive power than scientists had thought. “If this is a one-in-a-hundred-year impact,” says Mark Boslough, of the Sandia National Laboratory in New Mexico, a co-author of one of the Nature papers, “we shouldn’t have seen it.” That’s because the odds are overwhelming that it would have occurred in a remote area, completely out of sight—which is more or less what happened with the Tunguska event in 1908, when a much larger object exploded in a sparsely populated area of Siberia. That one released about 1,000 Hiroshimas’ worth of energy, but it mostly flattened trees, and had no eyewitnesses—or at least, none who lived to tell the tale.
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The fact that the more recent rock did find its way to Chelyabinsk might be simple bad luck, but it raises the odds that other meteors are hitting unpopulated areas too. Boslough estimates that these smaller but still potentially devastating impacts could be ten times more likely than scientists have assumed.
The next huge impact, comparable to the one that wiped out the dinosaurs 65 million years ago, is probably not coming for millions of years, and a rock like that, up to six miles (9.6 km) across, will at least be much easier to spot far in advance. In the meantime, Chelyabinsk should serve as fair warning that finding and cataloging the larger population of much smaller space rocks is more urgent than we ever knew.
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