Gravity tractor
A gravity tractor is a theoretical spacecraft that would deflect another object in space, typically a potentially hazardous asteroid that might impact Earth, without physically contacting it, using only its gravitational field to transmit the required impulse. The gravitational force of a nearby space vehicle, though small, is able to alter the path of a much larger asteroid if the vehicle spends enough time close to it; all that is required is that the vehicle thrust in a consistent direction relative to the asteroid's path, and that neither the vehicle nor its expelled reaction mass come in direct contact with the asteroid.