Let's say we detect an asteroid about to hit Earth and for some reason we can't throw Bruce Willis at it. (My problem-solving strategies generally involve throwing Bruce Willis.) Well, a swarm of tiny spacecraft could also do the job.
That's the finding of Alison Gibbings and Massimiliano Vasile, a pair of aerospace engineers at the UK's University of Strathclyde, which has got to be the most awesomely British placename I've heard all week. Their idea is to send a swarm of spacecraft, all roughly the size of a pebble, at an oncoming asteroid.
According to New Scientist, 500 kilograms worth of these spacecraft could push a 250-meter wide asteroid at least 35,000 kilometers off course, which would be more than enough to remove pretty much any chance of a collision with Earth. The swarm of spacecraft could be ferried from Earth to the asteroid in one big rocket. Once there, the swarm could form a sort of cloud-like shape around the asteroid, running off solar power and slowly guiding the asteroid off course.
Of course, you might wonder just why this is preferable to the obvious other solution, which is just taking that larger rocket used to bring the swarm there and firing it into the asteroid like a missile. The problem with using a single larger rocket as an explosive is that this generates smaller but still deadly asteroid chunks, any of which could still be headed towards Earth. The only drawback to this plan is that we'll need plenty of warning - for the swarm to really be effective, they would need to start work at least eight years before the estimated date of collision.