Toll from meteor strike is biggest ever

A meteor strike on Friday that injured almost a thousand people in a central Russian city inflicted the biggest known human toll from a space rock, experts said.

But the shock event has no link with a flyby by a rogue asteroid, they added.

"I am scratching my head to think of anything in recorded history when that number of people have been indirectly injured by an object like this," said Robert Massey, deputy executive secretary of Britain's Royal Astronomical Society (RAS).

"It's very, very rare to have human casualties" from a meteorite, he said.

Every day, around 100 tonnes of space debris -- mainly the rubble left from the building of the Solar System -- collides with Earth, say astrophycists.

The vast majority of it is dust and tiny pebbles, which burn up harmlessly at high altitude as they rub against the atmosphere, appearing in streaks of light called meteors that can often be seen on a clear night.

At the other end of the scale are kilometers- (miles-) wide behemoths of the kind that ended the reign of the dinosaurs some 65 million years ago, smashing into Earth with the force of an arsenal of nuclear weapons.

But these "geo-cruisers" pose only a very remote threat, occurring once over a span of millions of years, or even more, in statistical terms.

More common -- but still rare -- are larger objects that are metres or tens of metres (yards) across.

They survive the early stage of descent before exploding in the lower atmosphere, causing a shockwave, which is what happened on Friday, said Brigitte Zanda, a meteorite specialist at the Museum of Natural History in Paris.

Christophe Bonnal, head of rocket launchers at France's National Centre for Space Studies (CNES), said that, in this category, 85 percent of such objects fall in the sea "and of the remaining 15 percent, four-fifths fall in the desert or forests."

"So the probability of it landing on a city right at the moment when there are lots of people, really is extremely rare."

According to the Russian authorities, around 950 people were injured in Chelyabinsk, mainly by glass from shattered windows.

Massey, basing his estimate on news reports, said Friday's object was in all probability less than 10 metres (30 feet) across before it collided with Earth.

He, like other experts, said he saw "absolutely no connection" with asteroid 2012 DA 14, which was to skim the Earth on Friday at a distance of around 27,700 kilometres (17,200 miles).

Estimated to be 45 metres (150 feet) across, 2012 DA 14 would zip by within the orbit of geostationary satellites, making it the closest known flyby by a space rock but posing no threat.

"It (the Chelyabinsk event) happened 12 hours earlier, and that amounts to half a million kilometres (300,000 miles) of travel, (and) it seems to have been travelling in a different direction -- east-west, whereas the asteroid tonight will be travelling south to north," said Massey.

Said Bonnal: "Not on the same orbit, not on the same trajectory."

Stephen Lowry, an astronomer at Britain's University of Kent, predicted a keen search for remnants.

Meteorites can have great commercial value for collectors, but are especially prized by scientists, he said.

"We can learn quite a lot from them. We can see what this asteroid was made of and if we can reconstruct its pre-impact trajectory, we can get an idea of where it came from," he said.

The clues come from the rock's chemical signature by a machine called a spectrograph and from its internal structure, which is revealed by gently cutting it open.

Most meteorites come from rocks that have been jostled out of the asteroid belt, located between Mars and Jupiter, said Lowry.

But there are others that are knocked off the surface of Mars or the Moon by an impact and, after aeons in space, land on Earth. Another rarity is meteorites from comets -- primeval clusters of ice and dust.

But "at the moment, this one looks like a small, loosely packed asteroid body," said Lowry.