Hazardous asteroids may be more numerous than previously thought, scientists say

There are scads of building-size, potentially hazardous asteroids lurking in Earth’s immediate neighborhood, and they may be colliding with the planet 10 times as often as scientists have previously believed, according to new research published Wednesday that examined the airburst of a 25-million-pound asteroid this year near the Russian city of Chelyabinsk.

Three studies released Wednesday, two in the journal Nature and one in the journal Science, provide the most detailed description and analysis of the dramatic event on the morning of Feb. 15.

Scientists now estimate the diameter of the object at just a hair under 20 meters, or about 65 feet. Undetected by astronomers, the rock came out of the glare of the sun and hit the atmosphere at 43,000 mph.

As it descended through the atmosphere, it broke into fragments, creating a series of explosions with the combined energy of about 500 kilotons of TNT, making it more than 30 times as powerful as the atom bomb that destroyed Hiroshima in 1945, although the energy in this case was spread out over a much broader area.

The shock wave blew out windows in nearly half the buildings in Chelyabinsk. It knocked people off their feet; dozens were sunburned by the blinding flash, which at its peak was 30 times as bright as the sun. About 1,200 people were hurt, most by broken and flying glass, but no one was killed.

To study the meteorite fragment and its internal components (including mineral composition), a 0.53g fragment of the Chelyabinsk meteor, which exploded over Russia on Feb. 15, 2013, was imaged using X-ray tomography. This work was done by Douglas Rowland (Center for Molecular and Genomic Imaging) and Qing-Zhu Yin (Department of Earth and Planetary Sciences) at University of California, Davis. (Video courtesy of Science/AAAS)

One chunk the size of a love seat landed in frozen Chebarkul Lake and left a cosmic bullet hole in the ice. That fragment, which weighed about 1,900 pounds, was retrieved months later, breaking into several pieces in the process. Thousands of smaller pieces have also been recovered.

The scientific investigation relied to a great degree on video imagery obtained by “dash cams,” the cameras Russian drivers often use to document car crashes and potentially abusive law enforcement officers. Scientists visited 10 locations where the footage had been taken by stationary cameras and used landmarks to map the asteroid’s trajectory. The damage of the shock wave propagated perpendicularly to the rock’s path.

“It’s incredible how well documented all this is,” said Peter Jenniskens, who is a meteor astronomer at the SETI Institute and a co-author of the paper in Science.

Taken together, the new information on Chelyabinsk does not suggest that the sky is falling (no one has ever been killed by an asteroid in all of recorded human history).

But it may shift the overall risk profile of asteroids, making Chelyabinsk-size events look more probable.

That’s the conclusion of a new Nature report, “A 500-kiloton airburst over Chel­yabinsk and an enhanced hazard from small impactors.”

The scientific orthodoxy held that a Chelyabinsk-size event ought to happen roughly once a century. But Peter Brown, a professor at Western University in London, Ontario, and his collaborators reexamined decades of data compiled by government sensors. They saw several events that seemed to defy the odds.

Measuring the Chelyabinsk asteroid.

Famously, a very large object exploded over the Tunguska region of Siberia in 1908. There have been less-heralded impacts, including one on Aug. 3, 1963, when an asteroid created a powerful airburst off the coast of South Africa.

“Any one of these taken separately, I think you can dismiss as a one-off. But now, when we look at it as a whole, over a hundred years, we see these large impactors more frequently than we would expect,” Brown said.

NASA’s asteroid experts said they weren’t ready to recalibrate their estimates of impact frequency just yet. Paul Chodas, a scientist at NASA’s Jet Propulsion Laboratory who works on near-Earth objects, noted that on the same day that the Chelyabinsk asteroid hit, another asteroid — completely unrelated to the first — nearly hit the planet, too.

“We had two very rare events the same day, and immediately one can start asking questions about whether these are as rare as we thought,” Chodas said at a news conference Wednesday. “But I would point out that these are still small-number statistics.”

His colleague Don Yeomans, also of the Jet Propulsion Laboratory, echoed that point: “I would be more comfortable if that conclusion were backed up with more data.”

Most rocks in the Chelyabinsk range have yet to be identified, and it would be difficult and expensive to find them and calculate their trajectories, Brown said. But this could spur the development of new space telescopes and other early-warning systems, he said.

The paper in Science hypothesized that the Chelyabinsk asteroid is a piece of “rubble” from a larger body that had been broken apart by tidal forces from an earlier near-Earth encounter.

“The rest of that rubble could still be part of the near-Earth object population,” the authors wrote.

Joel Achenbach writes on science and politics for the Post's national desk and on the "Achenblog."
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