A suicide mission to an asteroid tests a way of defending Earth

that Earth is threatened from outer space sounds, on first encounter, like science fiction. Potentially hostile aliens either do not exist or, if they do, are too far away to matter. But space rocks are real—and some, at least, are too close for comfort. It is less than a decade since residents of Chebarkul, a city in Chelyabinsk oblast, Russia, witnessed the explosion in the atmosphere of a meteorite reckoned to have been a mere 20 metres across. Though no one was killed in this incident, about 1,500 people were seriously injured, mostly by flying window glass. If Earth were hit by something a bit bigger than the Chelyabinsk bolide, the damage could be immense. And there are a lot of candidates out there. The Observations Programme, a project intended to locate and track so-called Near Earth Objects that are 140 metres across or bigger—of which there are reckoned to be about 25,000—is less than halfway to its goal of cataloguing 90% of them. So far, no immediate threat has been discovered. But if one were to be, would there be anything people could do about it?