Ion Beam Shepherd for Contactless Asteroid Deflection

   A novel slow push asteroid deflection strategy has been recently proposed [1,2] in which an Earth threatening asteroid can be deflected by exploiting the momentum transmitted by a collimated beam of quasi-neutral plasma impinging against the asteroid surface. The beam can be generated with state-of-the art ion engines from a hovering spacecraft with no need for physical attachment nor gravitational interaction with the celestial body. The spacecraft, placed at a distance of a few asteroid diameters would need an ion thruster pointed at the asteroid surface as well as a second propulsion system to compensate for the ion engine reaction and keep the distance between the asteroid and the shepherd satellite constant throughout the deflection phase.

   Among the "slow-push" asteroid deflection methods the Gravity Tractor is regarded as one of the most promising, although the required mass is a major drawback. The Ion Beam Shepherd concept (IBS) promises to overcome the limitations of the Gravity Tractor concept by using the momentum of impinging propellant ions rather than gravity to achieve a contactless slow-push deflection, whose magnitude does not depend on the spacecraft and asteroid mass but rather on the  characteristics of the power and propulsion system employed. Low-divergence ion thrusters tested in  previous space missions can be used for this purpose without the need of substantial modifications, although an improvement in terms of specific impulse and beam divergence would increase the IBS capability.