On February 15 of this year, the Earth dodged a bullet of cosmic proportions as a meteoroid exploded over Chelyabinsk, Russia with the force of a nuclear weapon. Last Friday, NASA announced a new report published in Science that used videos and eyewitness accounts to provide new insights into the incident and the nature of the object that caused it.

The Chelyabinsk incident wasn't just another shooting star. It was the closest call the Earth has had since the Tunguska impact in 1908 made a mess of a large part of Siberia. Last February, a fireball brighter than the Sun streaked across the morning sky in a spectacle seen by thousands, recorded by dashboard cameras all over the region, and was heard by scientific instruments around the world.

When it exploded, the 500 kiloton blast was 40 times the explosive force of the Hiroshima atomic bomb. The detonation broke windows and damaged buildings. In all, 1,491 people, including 311 children, were injured, 70 cases of flash blindness were reported, and the fireball put out so much UV radiation that many people suffered from sunburn.

Shortly after the incident, a team led by Olga Popova of the Institute for Dynamics of Geospheres of the Russian Academy of Sciences in Moscow and meteor astronomer Peter Jenniskens of NASA’s Ames Research Center and the SETI Institute assembled 59 researchers from nine countries to try to sort out what had happened and what had caused it.

"Our goal was to understand all circumstances that resulted in the damaging shock wave," says Jenniskens. "Based on infrasound data, the brightness of the fireball, and the extent of the glass damage area, we confirm that this event was 100 times bigger than [the 2008 Sutter’s Mill meteorite fall in California]."

The team visited over 50 villages, talked to witnesses, selected the ten best videos, and did on-site measurements of angles of the meteor’s flight. From these, they calculated the trajectory by matching the path of the meteor against stars visible in the videos.

From the calculations and other data, the team constructed a map showing blast damage along a 55-mile (90-km) wide path. According the report, the meteor entered Earth's atmosphere at over 42,500 mph (19 km/s). As the fireball formed at an altitude of 19 miles (30 km), it was brighter than the Sun and the energy of the passage and explosion was distributed at various altitudes, which contributed to the damage on the ground.

The meteor shattered into a cloud of debris that fell over an area from Aleksandrovka to Deputatskiy and Timiryazevsky, with the largest bits hitting Lake Chebarkul, where they made a 22-ft (7-m) hole in the ice. The impact was captured on video by a security camera – the first time a meteor strike has been recorded live. This allowed scientists to calculate the impact speed of about 500 mph (225 m/s).

Ural Federal University researchers, led by meteoriticist professor Viktor Grokhovsk, set out to find what was left and eventually, with the help of professional divers, recovered a 1,400 lb (650 kg) meteorite.

Based on the evidence, the team estimates that the Chelyabinsk meteoroid was 20 m across and that 9,000 to 13,000 lb (4,000 to 6,000 kg) of it reached the ground, though this only comprised 0.03-0.05 percent of the initial mass. In all, 76 percent of meteoroid evaporated or was blasted to dust. When it exploded, the fireball was so hot it glowed orange, forming a mushroom cloud.

"The reason so little survived is that the radiation was so intense it contributed to evaporating the fragments before they could fall as meteorites out of this cloud," says Popova.

The odd thing about the meteorite fragments is, they didn't dig very far into the Earth when they hit. If this was an old Quatermass serial, that would be worrying, but NASA says that this had more to do with the history of the meteoroid. On analysis, the team found that the fragments contain a layer of small iron grains at each rim called shock veins, which had precipitated out of the glassy material when they cooled.

"One of these meteorites broke along one of these shock veins when we pressed on it during our analysis," says NASA Ames meteoriticist Derek Sears.

These veins were caused when the meteoroid was the victim of asteroid impacts, perhaps 4.4 billion years ago.

"Impacts on the parent body fractured the rock and pushed molten metal and metal sulfides through the cracks," says Cosmochemist Mike Zolensky from NASA’s Johnson Space Center in Houston. "There are cases where this increased a meteorite's mechanical strength, but Chelyabinsk was weakened by it."

From the data collected, Jenniskens could calculate the orbit of the Chelyabinsk meteoroid and says that it probably came from the Flora asteroid family in the asteroid belt between Mars and Jupiter. He speculates that Chelyabinsk was part of a larger rubble pile asteroid that broke up 1.2 million years ago during an earlier flyby of the Earth.

The team’s report has been published in Science.

Source: NASA

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