It was the biggest asteroid to strike Earth in more than a century, yet no one saw the Chelyabinsk meteor coming.
When the space rock smashed into western Russia in February 2013 it generated a shockwave as strong as 35 Hiroshima atomic bombs, leaving more than 1,600 people injured.
But how come no one detected the 60ft (19 metres)-wide meteor heading straight for us? The answer, experts say, is that it was hidden by the glare of our sun.