The asteroid that smashed into the central Russian city of Chelyabinsk initially measured 19 metres (61 feet) across, packing the energy of dozens of Hiroshima bombs, a study said on Wednesday.

Scientists in the Czech Republic and Canada analysed video and audio footage and fragments recovered from the dramatic incident on February 15.

They estimated the asteroid had probably once been part of the same, massive celestial object as a two-kilometre (1.2-mile) behemoth called 86039 -- a nasty "geocruiser" first spotted in 1999 that regularly comes close to Earth's orbit.

On entering Earth's atmosphere, the asteroid weighed 12,000 tonnes, a mass translating into the energy equivalent of 500,000 tonnes of TNT, according to the paper in the journal Nature.

This is roughly equivalent to between 27 and 41 times the explosive yield of the first atomic bomb, used on Hiroshima in 1945.

"The asteroid broke into small pieces between the altitudes of around 45 and 30 kilometres (28 and 18 miles), preventing more serious damage on the ground," says the study.

"The total mass of surviving fragments larger than 100 grammes (3.5 ounces) was lower than expected."

Previous estimates have put the object at 17 metres (56 feet) across and a mass of around 10,000 tonnes.

About 1,200 people were hurt by the shockwave, which blew out windows and damaged buildings across five Russian regions.

According to an Internet survey published in the US journal Science, 25 people who were outside at the time said they received sunburns from the ultraviolet light released by the meteor.

The study also says the object was a so-called LL chondrite type of rock, which is believed to constitute a minority in the asteroid belt.

It is of the same type as the asteroid Itokawa, samples of which were collected by the Japanese mission Hayabusa.

In a separate study also published by Nature, a team led by University of Western Ontario researcher Peter Brown carried out a survey of big airbursts by meteors.

The number of big asteroids could be "an order of magnitude" -- tenfold -- higher than estimated by telescopic observation of the space around Earth, it said.

 

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