Dr Edward Lu and Dr Stanley Love propose that a rocket be launched into space, effectively to act as a giant magnet. The gravity tractor would hover alongside the asteroid, with its thrusters pointing outwards so the exhaust does not affect the surface. The tractor would then pull the asteroid off course, merely using the gravitational pull between the two bodies. The scientists calculate that, with sufficient warning, a 20 tonne gravity tractor could safely deflect an asteroid 200 metres across in about a year of towing.