Gravity Tractor

Gravity pulls any two objects in space towards each other. Even a very small object like a spacecraft pulls on a large object like an asteroid. The asteroid pulls the spacecraft much stronger, because it's mass is much higher. If the spacecraft pushes away from the asteroid gently enough, the asteroid will follow, without changing the relative distance. Given enough time (several years), a low-thrust spacecraft can change an asteroid's orbit enough to prevent the asteroid from hitting the Earth without touching it. Solar sails and electric thrusters are low-thrust propulsion systems well-suited to use as gravity tractors. Wie2007:Wie2008b:Fisher2008