Although space scientists are confident that they have mapped the majority of "near Earth objects" that could conceiveably collide with us, and found that we're safe for now, there's always the possiblity that something unexpected might happen. Like the asteroid Apophis, which will slip past the Earth in 2029, and then make a return visit in 2036. If it alters its course there is a remote one in forty-five thousand chance that it could hit us.
Thankfully, University of Rome La Sapienza researcher Daniele Fargion has come up with a new strategy to tackle the problem head on - a nuclear powered rocket drill which burrows into the object and hurls the rock it digs out into space, pushing the asteroid off its Earth-bound course. This is an application of Newton'sThird Law - for every action there is an equal and opposite reaction. Flinging the drilled-out debris into space will give the object a kick in the other direction. Fargion's calculations show that over a ten year period this could deflect a 1 kilometre asteroid by upto 30,000 kilometres, enough to miss the Earth. But it's not all plain sailing. Asteroids are often loose aggregations of rocks and debris, which may prove difficult to drill into. Another problem is that they are also very often spinning, so the system would have to be programmed to spit out rocks only when the asteroid was pointing in the right direction. Undeterred, however, Fargion proposes testing the idea on our own moon, where his "screw rockets" could help to dig out underground shelters for future human use.