Chelyabinsk meteorite
© Qing-zhu Yin
Slice of future shock: A fragment from the meteorite shows numerous veins from a long-ago impact shock that weakened the original object.
Consumer video cameras and advanced laboratory techniques gave scientists an unprecedented opportunity to study the meteor that exploded over Chelyabinsk, Russia, in February. The explosion was equivalent to about 600 thousand tonnes of TNT, 150 times bigger than the 2012 Sutter's Mill meteorite in California.

"If humanity does not want to go the way of the dinosaurs, we need to study an event like this in detail," says Qing-zhu Yin, professor in the department of earth and planetary sciences at University of California, Davis.

Saying it was a "wake-up call," Yin says the Chelyabinsk meteorite, the largest strike since the Tunguska event of 1908, belongs to the most common type of meteorite, an "ordinary chondrite." If a catastrophic meteorite strike were to occur in the future, it would most likely be an object of this type.

"Our goal was to understand all circumstances that resulted in the damaging shock wave that sent over 1,200 people to hospitals in the Chelyabinsk blast area that day," says Peter Jenniskens, meteor astronomer at SETI Institute.
Their findings are published in the journal Science.

Based on viewing angles from videos of the fireball, researchers calculated that the meteoroid entered Earth's atmosphere at just over 19 kilometres per second, slightly faster than had previously been reported.

Sunburn

"Our meteoroid entry modelling showed that the impact was caused by a 20 metre-sized single chunk of rock that efficiently fragmented at 30 km altitude," says Olga Popova of the Russian Academy of Sciences in Moscow. The meteor's brightness peaked at an altitude of 29.7 km (18.5 miles) as the object exploded. For nearby observers, it briefly appeared brighter than the sun and caused some severe sunburns.

The team estimated that about three-quarters of the meteoroid evaporated at that point. Most of the rest converted to dust and only a small fraction (4,000 to 6,000 kilogrammes, or less than 0.05 per cent) fell to the ground as meteorites. The dust cloud was so hot, it glowed orange. The largest single piece, weighing about 650 kilogrammes, was recovered from the bed of Lake Chebarkul in October by a team from Ural Federal University led by Professor Viktor Grokhovsky.
(A meteoroid is the original object; a meteor is the "shooting star" in the sky; and a meteorite is the object that reaches the ground.)

Widespread Damage

Shockwaves from the airburst broke windows, rattled buildings and even knocked people from their feet. Popova and Jenniskens visited over 50 villages in the area and found that the shockwave caused damage about 90 kilometres (50 miles) on either side of the trajectory.

The shape of the damaged area can be explained by the fact that the energy was deposited over a range of altitudes.
The object broke up 30 kilometres above under the enormous stress of entering the atmosphere at high speed. The break-up was likely facilitated by abundant "shock veins" that pass through the rock, caused by an impact that occurred hundreds of millions of years ago. These veins would have weakened the original meteoroid.

Yin's laboratory carried out chemical and isotopic analysis of the meteorites and Ken Verosub, professor in the department of earth and planetary sciences, measured the magnetic properties of metallic grains in the meteorite. Doug Rowland, project scientist in the Center for Molecular and Genomic Imaging in the department of biomedical engineering, contributed X-ray computed tomography scanning of the rock.

Violent History

Put together, these measurements confirmed that the Chelyabinsk object was an ordinary chondrite, 4,452 million years old, and that it last went through a significant shock event about 115 million years after the formation of the solar system 4,567 million years ago. That impact was at a much later date than in other known chondrites of the same type, Yin says, suggesting a violent history.

Jenniskens calculates the object may have come from the Flora asteroid family in the asteroid belt, but the chunk that hit the Chelyabinsk area was apparently not broken up in the asteroid belt itself. Researchers at the University of Tokyo and Waseda University in Japan found that the rock had been exposed to cosmic rays for only about 1.2 million years, unusually short for rocks originating in the Flora family.

Chelyabinsk belonged to a bigger "rubble pile" asteroid that broke apart 1.2 million years ago, possibly in an earlier close encounter with Earth, Jenniskens speculates. The rest of that rubble could still be around as part of the near-earth asteroid population.

Major meteorite strikes like Tunguska or Chelyabinsk occur more frequently than we tend to think, Yin says. For example, four tonnes of material were recovered from a meteor shower in Jilin, China in 1976. "Chelyabinsk serves as unique calibration point for high energy meteorite impact events for our future studies."