We have seen a smaller version of this nightmare scenario more recently. In 2013, the Chelyabinsk Meteor, which disintegrated at an altitude of 30 kilometres, shattered windows and injured 1,400 people in the western Russian city. It delivered an explosion equal to about 500 kilotons – about 30 or so Hiroshima bombs – though it exploded high enough to not cause too much structural damage. That kind of impact is much more common, occurring about three times per year. Most occur over the ocean or in remote areas, so we usually don't notice. But the question isn't if an impact will happen, but when.

Governments are taking this seriously; taking the first tentative steps to preventing a dangerous impact. In January, Nasa formed the Planetary Defense Coordination Office to act as a clearing house for asteroid observations and work with other space agencies to discuss how large space rocks on a collision course with Earth might be dealt with.

Right now, the PDCO spends much of its effort in detection, coordinating various observation programs, says Lindley Johnson, Nasa's planetary defense officer. That's because you can't deal with the space rocks until you know where they are. "We try to detect anything that might be a threat years, if not decades, in advance," he says. Once a dangerous asteroid is identified, the actual plans for stopping one enter the picture.

The simplest method is a kind of planetary billiards, using a space probe to send a heavy object (or the probe itself) smashing into it. The asteroid will then, hopefully, be pushed off course and miss the Earth.