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meteor trail
The trail left by the meteor in the skies above Chelyabinsk, Russia, on February 15. (credit: Wikimedia Commons)

Piecing together the Chelyabinsk event

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On the morning of February 15, a previously-undiscovered object nearly 20 meters in diameter streaked into the atmosphere above Russia, exploding a couple dozen kilometers above and due south of the Russian city of Chelyabinsk. The blast wave from the airburst shattered windows in the city and surrounding region, injuring more than one thousand people, but fortunately killing none (see “Skyfall: will a Russian meteor and an asteroid flyby change our minds about the NEO threat?”, The Space Review, February 18, 2013).

“The people in Chelyabinsk and surrounding villages were lucky that this came in at such a shallow angle,” said Boslough; a steeper angle would have enhanced the shock wave by an order of magnitude.

Two months later, planetary scientists are gaining a better understanding of what happened that day. At the opening session of the Planetary Defense Conference 2013 Sunday night in Flagstaff, Arizona—a meeting scheduled long before the Chelyabinsk event—a panel of researchers discussed what they’ve learned about what many called a “once in a lifetime” event from the limited data available, and its implications for keeping track of, and defending the Earth from, near Earth objects.

One ongoing challenge is to try and determine just how energetic the airburst was. Early estimates put the energy of the meteor at as high as 500 kilotons of TNT, but Peter Brown of the University of Western Ontario said that different techniques yield different results. Infrasound stations measured the pressure wave as it propagated through the Earth’s atmosphere, producing signatures not seen since the last aboveground Chinese nuclear tests in the 1970s. “The ringing in the atmosphere lasted for several days,” he said. Analysis of the data results in an estimated energy of 600 kilotons of TNT. Seismic stations also detected the blast wave as it hit the ground. Models based on that data result in a lower energy, of 425 kilotons of TNT, but with an uncertainty of 200 kilotons.

Brown and his colleagues have also been taking advantage of a unique aspect of the Chelyabinsk event: the hundreds of videos of the meteor taken by security cameras and “dashcams” mounted on cars. He said his team has cataloged and geolocated nearly 400 videos from Chelyabinsk, about half of which show the meteor or the flash of light from the fireball. They have calibrated those observations to get the trajectory and velocity of the meteor and the peak brightness of the fireball. Those models give an estimate of 415 kilotons for the airburst.

Given those varying estimates, Brown said the best estimate available for the energy of the airburst is about 500 kilotons, but with an uncertainty as large as 50 percent. That implies that the size of the meteor, estimated to be about 17 meters in diameter, could be as large as 21 or 22 meters, he said.

Brown also said that, based on modeling of the meteor, a remnant weighing up to 200 kilograms may have survived to the ground, breaking through the ice of Lake Chebarkul, although no large fragments have yet been recovered from the lake.

Given the meteor’s energy, the effects of the airburst could have been much worse. A meteor streaks to earth much faster than a nuclear weapon—about 17.5 kilometers per second in the case of the Chelyabinsk meteor—which drives the fireball and blast wave down in a column, resulting in greater destructive effects than a nuclear explosion of the same yield at the same altitude. “On a megaton for megaton basis, they’re more damaging,” said Mark Boslough of Sandia National Lab.

“This asteroid could not have been optically detected from the ground” because of its size and orbit, Chodas said.

Chelyabinsk lucked out, though, since the track of the meteor was relatively shallow: about 16 degrees. Had the Chelyabinsk meteor come in at a steeper angle—say, 16 degrees to the vertical—the shock wave effects would have been an order of magnitude stronger, Boslough estimated. “The damage on the ground at ground zero would have been much more serious,” he said. “The people in Chelyabinsk and surrounding villages were lucky that this came in at such a shallow angle.”

In addition to dashcams and other videos, researchers are augmenting their data on the meteor with ground truth. Peter Jenniskens of the SETI Institute spent two weeks in Chelyabinsk and the surrounding region, working with local researchers to document the effects of the meteor as much as possible.

When he arrived in the city, three weeks after the meteor, he found to his surprise—and dismay—that much of the damage had already been repaired, or was in the process of doing so, like a zinc factory whose roof collapsed. “I could have seen this as a fantastic science museum,” he said.

Jenniskens said they traveled through the countryside, visiting communities and talking with locals who witnessed the meteor and, in some cases, collected meteorite fragments from it. “Our strategy would be to go to markets in these villages, because the market people were the ones hearing all the rumors” about the meteor, he said. One person they talked with, Jenniskens said, claimed that he suffered a sunburn from the fireball—briefly brighter than the sun—strong enough that his skin peeled afterwards.

The meteor was not seen before it exploded over Chelyabinsk because of both its small size and its orbit: it was coming towards Earth from the direction of Sun and thus could not be seen. Paul Chodas of JPL said they have worked out an orbit for the object based on the meteor track, showing the object was in an elliptical orbit that went from the main asteroid belt to inside the orbit of the Earth.

That orbit, he said, shows that the object didn’t pass close enough to the Earth in the preceding 30 years or so that it could have been seen, thus ruling out any “pre-discovery” images that could refine its orbit. “This object would not have been much brighter than 25th magnitude,” he said, too dim to be seen unless it passed close to the Earth, which it didn’t over the last few decades. “This asteroid could not have been optically detected from the ground.”

Chodas added that there are many more bodies the size of the Chelyabinsk meteor out there: approximately nine million near Earth objects (NEOs) about 18 meters in diameter. By comparison, there are about 500,000 objects the size of 2012 DA14, an object about 45 meters across that coincidentally flew close to the Earth the same day.

Finding these smaller ones, though, is difficult for current search techniques, because they are so faint. “Finding the small ones is the challenge the asteroid searchers will be facing,” he said. However, current searches still have plenty of work to do to achieve the current goal of finding 90 percent of NEOs greater than 140 meters in diameter. “The 18-meter size is really, really difficult.”

“The most important lesson for me for the Chelyabinsk impact is that it did not set off a nuclear war,” said Morrison.

To some, the Chelyabinsk event was a warning that it was time to devote more attention to smaller objects, as opposed to those 140 meters in diameter and larger. “We do have to worry about the small stuff,” Boslough said. “We don't have a system in place to discover objects this small on collision courses.” He advocated in particular for space telescopes that do not have the same sunward-direction blind spots as terrestrial telescopes.

“Chelyabinsk was a wakeup call for these small impacts,” Brown said.

Others in the conference session, though, worried that the event might draw attention away from those larger, and more dangerous, objects. “We’re greatly exaggerating the danger,” said David Morrison of NASA and the SETI Institute, noting that many people were killed in shootings or automobile accidents just during the time of the session, while Chelyabinsk killed no one. “It would greatly distort our true interest in surveying for near Earth objects in the 100, 200, 300 meter range that are the real danger if we try to divert our interest to worrying about a once-in-a century event that didn’t kill anyone.”

For Morrison, the key lesson from Chelyabinsk was what didn’t happen. For decades, he said, people feared that a meteor like Chelyabinsk might be interpreted as a nuclear strike that could trigger a “counterstrike” and an accidental nuclear war. But this time, he said, there was no evidence that Russian officials ever considered the meteor as a military strike. “The most important lesson for me for the Chelyabinsk impact is that it did not set off a nuclear war.”