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How rare was Chelyabinsk asteroid blast? Not as rare as you hoped. (+video)

Scientists have spent months researching the spectacular asteroid blast above Chelyabinsk last February. Among their findings: there are lots more of these objects out there than previously thought.

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February’s spectacular explosion of an asteroid above Russia, the most widely watched, data-rich collision in history between an asteroid and Earth, signals that the number of small asteroids with the potential to hit Earth likely is larger than previously estimated.

That’s the bad news. The somewhat better news is that the damage such asteroids can inflict if they burst high in the atmosphere, although significant, appears to be lower than scientists previously thought.

These are some of the conclusions scientists have reached after months of sleuthing to uncover the effects and possible source of a 62-foot-wide chunk of asteroid that burst apart high over the Russian region of Chelyabinsk.

After entering the atmosphere, the asteroid shed dust and smoke in a billowy cloud behind it. When it burst into pieces some 18 miles up, many on the ground felt the heat, which in some cases left people with an unexpected sunburn.

The shock wave it generated packed the punch of 500 kilotons of TNT, researchers estimate.

People immediately below the blast were knocked to the ground, while in the city of Chelyabinsk, 1,210 people were injured, mostly by flying glass, according to one of the analyses, conducted by an international team of 60 scientists from nine countries in North America, Europe, and Asia. The effort was led by Olga Popova, a researcher with the Russian Academy of Science in Moscow.

The object's mass ranged from 12,000 to 13,000 tons when it entered the atmosphere, Dr. Popova's team estimates. By the time the event was over, 76 percent of the object had vaporized. Virtually all of the rest was pulverized, forming dust. Only 4 to 6 tons of material struck the surface, including a 1,250-pound fragment that plunged into Lake Chebarkul. Researchers recovered it last month.

For all the damage and injuries it caused, “Chelyabinsk is now your gold standard” for estimating the damage that asteroid collisions can inflict, says Peter Jenniskens, a scientist with the SETI Institute and NASA's Ames Research Center in Mountain View, Calif.

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Until now, researchers had no well-measured event of comparable power to underpin their damage projections from hazardous space rocks, explains Dr. Jenniskens, a member of Popova's team.

Prior to February's encounter, an air blast over the forests of Tunguska in Siberia in 1908 was the most powerful in modern times. But aside from nearly 800 square miles of flattened forests, scientists had little to go on besides estimating the amount of explosives it would take to inflict the same damage and plugging that into damage models designed for nuclear weapons. [Editor's note: The original version missed by one year the date of the air blast in Siberia.]

Yet the physics behind the two types of blasts and the behavior of the shock waves are radically different, as scientists learned from Chelyabinsk.

“Once you understand Chelyabinsk, then you can consider steeper entries, or faster, bigger, or stronger objects” in models designed to better understand the potential damage an incoming asteroid could inflict – if it is detected.

In Chelyabinsk's case, no one saw the object coming. It followed a path any fighter pilot would appreciate: It came from the direction of the sun.

Indeed, small asteroids like the one that broke up over Chelyabinsk remain the bane of an asteroid hunters' existence. They are small and very hard to see under the best of circumstances.

Astronomers have discovered more than 90 percent of the estimated 1,100 near-Earth asteroids larger than a half mile across, according to a tally at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Researchers now are focusing their hunt on asteroids from 500 feet to half a mile across.

But as the size class of asteroids drop, their numbers increase dramatically. Astronomers so far have found 500 asteroids in a size range that would include Chelyabinsk's object and smaller, but they are thought to number in the millions.

How many million is unclear, notes Paul Chodas, a researcher at JPL.

At a briefing Tuesday, he noted that he and others had estimated that perhaps 3 million to 4 million objects of Chelyabinsk's size class may be out there as near-Earth asteroids. This estimate is based on telescope surveys that are trying to identify these smaller objects.

But the number could be substantially higher, according to one of the studies, published Wednesday, by a team led by Peter Brown of the University of Western Ontario. His team drew on small-asteroid encounters over the past few decades by satellites and a global network of sensors that pick up the sounds of asteroids when they burst.

Those events suggest that the likelihood that Earth will be smacked by an object as large or larger than the one that broke up over Chelyabinsk is as much as 10 times higher than that predicted by telescope surveys.

Dr. Chodas cautions that the estimate is based on a tiny sample. Still, he says, the number of Chelyabinsk-size asteroids “could be several times more” than previously estimated.

At the same time, however, the damage inflicted on the Chelyabinsk region was smaller than astronomers had predicted for the object's size. Prior to Chelyabinsk, they lacked the detailed understanding of what happens during an asteroid air-burst.

Fragments of the object that did reach Earth also have yielded a treasure trove of information that opens a window on the object's origin and on its demise over Chelyabinsk. The parent object was part of a larger asteroid that formed some 115 million years after the solar system was born, according to Popova's team. It likely got kicked out of the inner part of the main asteroid belt between Mars and Jupiter to become a near-Earth asteroid.

Chemical and isotopic studies of fragments from the Chelyabinsk event showed that the impactor broke away from the larger asteroid about 1.2 million years ago, during a close encounter with Earth. The break-up left the Chelyabinsk impactor in one piece. But it was laced with fissures fused by iron that melted from the heat of the impact and filled the cracks.

That was enough to hold the object together in space, but not enough to prevent the asteroid from shattering under the heat and pressure of its encounter with Earth's atmosphere, the researchers say.

When a third team, which included Brown, reconstructed the impactor's orbit, they found that its path was similar to that of asteroid 1999 (NC43), which is roughly 1.4 miles wide. The orbits are so similar that the team suggests they both came from the same parent object. The asteroid is on JPL's list of potentially hazardous asteroids. Next March it is due to swing within 3 million miles of Earth, but if you wait until 2104 it will pass within 316,000 miles of Earth, just 77,000 miles further out than the Moon.

Understanding the nature of the impactor and a having a virtual ring-side seat – thanks to dashboard video cameras in Russian cars posted to YouTube – at its break-up highlights the value of knowing how an asteroid is built in estimating what is likely to happen if it encounters Earth, researchers say.

Two of the research papers released Wednesday are set for publication on Thursday in the journal Nature. The third is set to appear Friday in the journal Science.

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