500 injured in Russian meteor crash

Witnesses reported a fireball streaking across the sky over the Ural Mountains, causing explosions.

It comes as sky-gazers are set to watch an asteroid as large as an Olympic swimming pool race past Earth tonight at a distance of 17,200 miles – the closest ever predicted for an object of that size.

It will pass closer even than satellites which orbit Earth, but there is no risk of collisions, experts say. The asteroid will be closest at 7.25pm. For regions in darkness around that time, it will be visible using good binoculars or a telescope.

The asteroid orbits the sun in 368 days, a period similar to Earth's year, but it does not orbit in the same plane as Earth. As it passes at 17,450 mph, it will come from "under" Earth and return back toward the sun from "above".

It will pass over the eastern Indian Ocean, making for the best viewing in Eastern Europe, Asia and Australia.

But keen viewers anywhere can find one of several live streams of the event by logging on to the internet, including a feed from the Jet Propulsion Laboratory at Nasa from 7pm.

The asteroid, called 2012 DA14, was first spotted last February by astronomers at the La Sagra Sky Survey in Spain.

They caught sight of the asteroid after its last pass at a far greater distance. From their observations, they were able to calculate the asteroid's future and past paths and to predict today's near-miss, which will be the closest the object comes for at least 30 years.

During today's fly-by, scientists will use radar to study DA14 and learn about its composition and structure.

The knowledge could prove useful if steps have to be taken to remove the threat of another space rock. The "Hollywood option" of blowing up an incoming asteroid has been ruled out by experts. Such a solution would only result in deadly debris raining down on Earth.

Instead, scientists are looking at ways of gently nudging an asteroid onto a safer trajectory. The American space agency Nasa has plans for a future mission called Dart which will fire a probe at an asteroid to see if it can be moved.