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. [1, 2, 3]

References

1. Wie, B. Hovering Control of a Solar Sail Gravity Tractor Spacecraft for Asteroid Deflection. 17th AAS/AIAA Space Flight Mechanics Meeting, Sedona, Arizona, AAS paper pp. 07--145, 2007. BibTeX
2. Wie, Bong. Kinetic Impactors and Gravity Tractors for Asteroid Deflection. Asteroid Deflection Research Symposium 2008, October 23-24 2008, Arlington, VA , 2008. BibTeX
3. Fisher, Jeff. Solar Reflector Gravity Tractor for Asteroid Collision Avoidance. Asteroid Deflection Research Symposium 2008, October 23-24 2008, Arlington, VA , 2008. BibTeX