Hovering Control of a Solar Sail Gravity Tractor Spacecraft for Asteroid Deflection∗

A solar sail gravity tractor (SSGT) spacecraft is proposed as a viable option for deflecting a certain class of near-Earth asteroids (NEAs) such as highly porous rubble piles rather than solid monolithic bodies. Solar sails are large, lightweight reflectors in space that are pushed by sunlight. The SSGT spacecraft concept is based on the so-called Gravity Tractor (GT) for towing asteroids by using gravity as a towline, which was proposed by Lu and Love in the 10 November 2005 issue of Nature. It exploits the “propellantless ” nature of solar sails; consequently, its probable advantage over a GT spacecraft propelled by ion engines is its longer mission life times (> 10 years) with a larger “propellantless ” ∆V capability. Furthermore, it has no concern of rocket plume impingement on the asteroid surface. This paper demonstrates the practical hovering control feasibility of an SSGT spacecraft for towing NEAs. For example, a 5-year towing of asteroid Apophis using a 2500-kg SSGT spacecraft (equipped with a modest 90-m, 50-kg solar sail of a 0.03-N solar thrust with a 35-deg sun angle) and an additional 3-year coasting time will result in an orbital deflection of 30 km in 2029. A 30-km deflection is more than sufficient to move Apophis out of its 600-m keyhole. I