The idea's been proposed and kiboshed before, but a team of British scientists and engineers are taking a look at gravity as one way to protect the Earth from asteroids.
NASA's Near Earth Object program currently has 145 potentially hazardous asteroids on its list out of 1,062 objects larger than one kilometer in diameter, and 6,292 total discovered objects. What's all that mean? Well, that there's a lot of stuff out there that could potentially impact our planet — some of it pretty big.
So instead of sending shuttle crews up at the last minute to blow an approaching asteroid up, British astronomers at the Astrophysics Research Centre are planning to build a 10-ton "gravity tractor" spacecraft that will influence the object's trajectory. The process would take some time — a craft would have to be launched 15 years in advance to really have an effect — but, once the tractor arrives, it'd hover close by an asteroid and gently guide it along a different path.
Besides the inordinate amount of time it'd take, the "gravity tractor" program — still in its early stages — would cost so much and require so much in terms of personnel that it would take either the backing of a government or several to ever see it through.