  The Gravity Tractor (GT) is a fully controlled asteroid deflection concept
using the mutual gravity between a robotic spacecraft and an asteroid to slowly
accelerate the asteroid in the direction of the "hovering" spacecraft. Based on
early warning, provided by ground tracking and orbit prediction, it would be
deployed a decade or more prior to a potential impact. Ion engines would be
utilized for both the rendezvous with the asteroid and the towing phase. Since
the GT does not dock with or otherwise physically contact the asteroid during
the deflection process there is no requirement for knowledge of the asteroid's
shape, composition, rotation state or other "conventional" characteristics. The
GT would first reduce the uncertainty in the orbit of the asteroid via Earth
tracking of its radio transponder while station keeping with the asteroid. If,
after analysis of the more precise asteroid orbit a deflection is indeed
indicated, the GT would "hover" above the surface of the asteroid in the
direction of the required acceleration vector for a duration adequate to
achieve the desired velocity change. The orbit of the asteroid is continuously
monitored throughout the deflection process and the end state is known in real
time. The performance envelope for the GT includes most NEOs which experience
close gravitational encounters prior to impact and those below 150-200 meters
in diameter on a direct Earth impact trajectory.
