Gravitational tractor

In asteroid deflection, a gravitational tractor is a way to use the gravitational attraction between a spaceship and an asteroid to modify its trajectory to prevent it from colliding with the Earth. In the plan, the spaceship hovers above the asteroid's surface using thrusters, and gravitational attraction deflects the asteroid. The thrusters must not point directly at the asteroid, else the effect would be voided by momentum transfer between exhaust gas and the asteroid's suface.

Writing in Nature, physicists Edward T. Lu and Stanley G. Love of NASA's Johnson Space Center show that a gravitational tractor is a reasonable proposition: it is robust to the asteroid's structure and rotation rate, and a technologically feasible plan (such as a 100 W nuclear electric propulsion system) could impart a delta v of a few millimeters per second to an asteroid with a diameter of 200 meters over the course of several years, sufficient to prevent an Earth impact.

References

Gravitational tractor for towing asteroids, Edward T. Lu and Stanley G. Love, [http://www.nature.com/nature/journal/v438/n7065/abs/438177a.html "Nature" 438 (10 November 2005), 177–178, doi:10.1038/438177a] ; also, see [http://arxiv.org/abs/astro-ph/0509595 astro-ph/0509595] in the arXiv.