russian meteor
© APThe meteorite fireball that fell over Chelyabinsk briefly burned 30 times brighter than the sun.
The meteorite is providing invaluable information to help protect against larger rocks that might pose a serious threat to Earth

Just after sunrise on 15 February 2013, as commuters made their way along snow-covered roads to Chelyabinsk in south-west Russia, the clear blue sky was torn by a hurtling lump of space rock.

The meteorite appeared without warning, out of the sun, on a shallow trajectory. It thumped into the atmosphere at 12 miles per second and became a fireball. For a moment, the rock burned 30 times brighter than the sun.

Viktor Grokhovsky, a researcher at Ural Federal University, 200km to the north of Chelyabinsk, missed the beautiful, terrifying spectacle that morning, but within minutes was watching video of the event. He spent the rest of the day assembling a search party. The first of several set out at first light the next morning to interview eyewitnesses and recover pieces of the fallen rock.

"It was rather easy to find fragments in the first days after the meteorite fell, because the chunks left holes in the snow," Grokhovsky told the Guardian. But as more snow fell over the next two weeks, the holes became covered over. The search was called off until the snow began to melt in the spring.

Nothing the size of the Chelyabinsk meteorite had fallen to Earth in 100 years, and never over an urban centre where its dramatic arrival would be captured by CCTV and dashboard video cameras installed by Russians wary of insurance scams and crooked police.

"This was the first time in modern, medieval or ancient history when a meteorite fell in an area with a high density population. This type of meteorite is rare and a lot of material fell. All these factors give excellent opportunities for extraterrestrial substance research. The hazard from asteroids and comets, people's behaviour in emergencies, and the shortage of astronomical education are all on the agenda," said Grokhovsky.

After the flash came the bang. The meteorite exploded with a force around 30 times that of the atom bomb dropped on Hiroshima, or 500 kilotonnes of TNT. The shockwave knocked people off their feet and shattered windows in thousands of apartments. The Earth rang to the blast, with vibrations picked up by seismic sensors 4,000km away.

Nobody was killed, but the blast injured more than 1,200 people. Many had cuts from flying glass. Others suffered retinal burns from watching the fireball, or burns that left their skin peeling. The toll was slight because the 20-metre-wide meteorite exploded so high, more than 20km above the ground.

Grokhovsky's search parties came back with more than 700 fragments of meteorite and scores of eyewitness accounts from locals. One of his expeditions had skied 50km, from village to village, beneath the meteorite's path. He used the information they gathered, along with public video footage, to piece together the trajectory of the rock and predict where much of it must have landed.

On the back of that work, a search party was sent to investigate a large hole that had appeared in ice covering a lake on the slopes of the southern Ural mountains. The team collected fragments of chondrite that matched other chunks of the meteorite. When Grokhovsky called on colleagues to measure the magnetic field at the lake, they found an anomaly in their readings near the hole. "Their analyses convinced us that exactly this meteorite had landed in the lake," Grokhovsky said.

It was October before a team of professional divers finally searched the lake, but the wait was worthwhile. "They found a 650kg meteorite fragment deep in the sludge," said Grokhovsky. The lump was so fragile it split in three when they weighed it. The heaviest chunk is now on display in the Chelyabinsk museum of local history.

"The most powerful impression for me personally was work on the night of 17 to 18 February. It was that moment when me and my colleagues from Ural Federal University's nanotech centre determined the meteorite's origin and the substance of the chunks which were collected not far from the hole in the ice," Grokhovsky said.

The recovery of so much of the meteorite will transform research in the field. Scientists can use the fragments to understand the rock's cosmic origins and its properties, all useful information to protect against a larger rock that might pose a serious threat to Earth in the future. Ominously, researchers have already discovered that there must be ten times as many potentially dangerous asteroids out there with sizes of the order of tens of metres as previously thought.

"It is very hard to overestimate the importance of the meteorite's recovery from the lake. The total mass of extraterrestrial substance that was pulled up weighed more than all the samples which were delivered to Earth by the [US] Apollo and [Soviet] Luna missions," said Grokhovsky.