Meteor explosions like this year’s Russian fireball more common than thought

Chelyabinsk-sized rocks may come to Earth every 30 years, on average

SPACE ROCKS This 570-kilogram meteorite was recovered from Lake Chebarkul on October 16.

Courtesy of Science/AAAS

Meteor impacts such as February’s explosion over Chelyabinsk, Russia, the most powerful observed in a century, may occur more frequently than thought. An analysis of recorded impacts over the past two decades suggests that Chelyabinsk-sized objects strike the planet every few decades, on average, rather than once every century or two.

“There were inklings of this before, but this is the strongest statement that’s been made,” says Paul Chodas, a planetary scientist with NASA’s Near Earth Object Program at the Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the research. If confirmed, scientists will need to reassess the risk of impacts and come up with new strategies for spotting space rocks tens of meters in diameter, which can cause widespread damage and injuries.

It’s impossible to find and track each of the millions of objects that cross Earth’s path. Instead, astronomers maintain a census of objects using ground-based telescopes and use those numbers to estimate how often asteroids of various sizes should strike the planet. According to those predictions, an impact like that of the 19-meter-wide Chelyabinsk meteor, which released the energy equivalent of about 500,000 tons of TNT, should take place roughly once in 120 years. Impacts as powerful as the famous Tunguska event of 1908, which was comparable to a 10-million-ton blast, should take place every few thousand years.

Concerned by multiple supposedly low-probability events having occurred over the course of a century, planetary scientist Peter Brown at Canada’s University of Western Ontario decided to analyze the energies of actual impacts recorded worldwide during the last 20 years. As expected, high-energy events corresponding to larger meteors occurred less frequently than less energetic ones. But the frequency did not drop off as quickly as telescope surveys predict. In a study published November 6 in Nature, Brown’s team shows that objects between 10 and 50 meters across strike Earth at least three times as frequently as expected. Chelyabinsk becomes a once-in-30-years event, while a Tunguska-like impact should occur once every few hundred years.

The largest surviving fragment of the February 15 asteroid tore a 7-meter-wide hole through the icy surface of Lake Chebarkul, which is about 70 kilometers from Chelyabinsk. Eduard Kalinin
“We need to try to find the source of the discrepancy,” Chodas says. He says the data suggest that either a subset of Chelyabinsk-sized objects have particularly Earth-threatening orbits, or telescope surveys severely underestimate the total number of those objects.

Clues may lie in two other studies also published November 6. In Science, researchers chemically analyzed Chelyabinsk meteorite fragments and suggest that the asteroid split off from a larger object as recently as 1.2 million years ago after a close encounter with Earth. Other researchers report in Nature that the orbit of a nearby asteroid, a 2,200-meter behemoth called 86039, is very similar to that of the Chelyabinsk asteroid. The two rocks may once have been part of the same object, the researchers conclude. Chodas says it’s possible, though far from proven, that a family of asteroids derived from one parent body might have Earth-threatening orbits.

Researchers in these two studies used videos from Russian citizens’ cell phones and car dashboard cameras to construct the asteroid’s trajectory. They calculate that the 12,000-metric-ton asteroid plunged into Earth’s atmosphere at more than 68,000 kilometers per hour; that makes it more than 25 times as massive as a fully loaded Boeing 747 and almost 75 times as fast. Friction from plowing through the atmosphere caused the rock to fracture about 30 kilometers above the surface, unleashing a fireball 30 times as bright as the sun and a shock wave aimed toward Chelyabinsk, an industrial city of over a million people.

The Chelyabinsk meteor shattered windows in more than 7,000 buildings and sent 112 people to hospitals within an area 180 kilometers from north to south and 80 kilometers from east to west. But the damage was not nearly as bad as scientists would have predicted for a 19-meter-wide object, Brown says. That’s because current damage predictions are based on explosions of nuclear bombs, he says; compared with a meteor, bombs fall from the sky much more slowly and release energy far more quickly.

Still, the meteor made a major impression on the people who saw it and felt it that February morning. Peter Jenniskens, an astronomer at the SETI Institute in Mountain View, Calif., and coauthor of the Science study, spent 16 days in March visiting 50 villages in the region, where he and colleagues interviewed hundreds of witnesses, assessed building damage and analyzed meteorite fragments. A coal miner reported that the fireball caused sunburn so severe that some of his skin flaked off. “Everybody we met had a story to tell,” Jenniskens says.

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