Imagine, if you will, a fairly large asteroid (>200 m) on a collision course with Earth. What do we do? Scientists could try to land a space tug on the surface and push the asteroid out if its devastating trajectory (assuming we have enough forewarning). We could launch an armada of nuclear warheads and hope that they are enough to break the asteroid into many small pieces, to burn up in re-entry. We could pull an Armageddon and land some crazy astronaut on the surface to drill and drop in a nuke to split the asteroid from the inside. Or, we could just curl up in a ball and weep. Edward Lu and Stanley Love have another idea: the gravitational tractor.
The idea is to have a space vehicle that hovers over the asteroid and uses gravity as a tow line to drag the asteroid to a safer orbit. No nukes, just a nuclear-electric propulsion system. The drawback? You need enough advance warning to launch and place the tractor in position. Still, a very cool idea and likely to see some advancement in the near future. I can even imagine using them to pull asteroids into a safe orbit to mine for various metals. If I get some time, I'll post about the Space Elevator or Australia's Solar Tower.