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System-level design considerations for asteroid despin via neutral beam emitting spacecraft

Asteroids have gained significant interest during the past several years in the areas of planetary defense, mining, and civil space. Hundreds of asteroids are discovered each year and that rate is increasing exponentially as technology improves. However, Chelyabinsk is a reminder that not every asteroid is observed and that the technology to prevent collision events has not been developed. Asteroids are not all likely to be monolithic boulders, but a large number are probably rubble piles held together by gravity and cohesion. To protect Earth, we must find ways to alter the trajectories of asteroids so that significant impact events never occur. Controlling smaller asteroids can be done through propulsive methods in proximity operation. However, as all asteroids spin, with some rotating on more than one axis, the thrust vector necessary to deflect an asteroid will likewise rotate. To thrust along the deflection vector for a long period of time, we must first despin the asteroid. This study investigates a novel concept for asteroid despin using spacecraft equipped with neutral beam emitters. While other methods for asteroid control employ vehicles that attach to the body, our proposed method only requires spacecraft to hover above the surface and induce a torque to stabilize the complex spinning behavior of asteroids. As the despin methods will cause considerable stresses inside the rubble pile, we present the maximum stress allowable before asteroid disaggregation due to Drukker-Prager failure calculated from gravitational and van der Waals (VDW) attraction. We provide relations for thrust and Isp as related to neutral beam application. The projected power required for a given beam energy is likewise presented to scale our system and concept. The time required to despin an example asteroid as dependent on force applied is also presented. In order to despin the asteroid to decrease the required power and time to despin, we then consider the possibility of multiple spacecraft working together. While there are several concepts for Earth defense in the event of a probable asteroid impact, most require a precursor mission to determine the size and composition of the asteroid to design systems that latch on or bag the body. Launch opportunities to Near Earth Asteroids can vary anywhere from years to decades making such concepts difficult to implement if there is a limited time frame from discovery to orbital characterization to the likelihood of Earth impact. Our concept requires no such precursor mission as there is no need for direct contact with the asteroid. This work discusses the system-level trades and requirements of an asteroid despin mechanism that shows distinct operational advantages when compared to other designs.