The effects of constrained electric propulsion on gravity tractors for planetary defense

Electric propulsion may play a crucial role in the implementation of the gravity tractor planetary defense technique. Gravity tractors were devised to take advantage of the mutual gravitational force between a spacecraft flying in formation with the target celestial body to slowly alter the celestial body's trajectory. No physical contact is necessary, which bypasses issues associated with surface contact such as landing, anchoring, or spin compensation. The gravity tractor maneuver can take several forms, from the originally proposed constant thrust in-line hover to the offset halo orbit. Both can be enhanced with the collection of mass at the asteroid. The form of the gravity tractor ultimately impacts the required thrust magnitude to maintain the formation, as well as constraints on the vectoring of the thrust direction. Solar electric propulsion systems provide an efficient mechanism for tugging the spacecraft-asteroid system due to their high specific impulse. Electric propulsion systems can generate thrust continuously at high efficiency, which is an ideal property for gravity tractors that may require years of operation to achieve the desired deflection because of the very low coupling force provided by the gravitational attraction. The performance and feasibility of the deflection are predicated on having the propulsion capability to maintain the gravity tractor. This paper describes the impacts of constraining the solar electric propulsion thrust magnitude and thrust vectoring capability. It is shown that uncertainty in asteroid density and size, when combined with the enforcement of the electric propulsion constraints, can preclude the feasibility of certain gravity tractor configurations. Additionally, odd thruster configurations are shown to drive the gimbal performance and to have major impacts on eroding incident spacecraft surfaces due to plume interaction. Center of gravity movement further exacerbates issues with gimbaling and plume interaction. A tighter plume divergence angle is therefore always desired, but this paper shows that there is an optimal momentum balance between plume interaction and asteroid-plume avoidance. Several gravity tractor techniques are compared based on metrics of time efficacy, as measured by the induced asteroid delta-V per unit time, and mass efficiency, as measured by the induced asteroid delta-V per unit mass of fuel. Given the propulsion constraints, halo orbits can be infeasible for smaller asteroids unless the mass of the spacecraft is augmented with collected material through a technique called the Enhanced Gravity Tractor. Another proposed method is to alter the halo period by canting the thrusters. In-line hover gravity tractors can always be moved along the net thrust direction to conform to the given propulsion system at the expense of performance, except in the case of smaller asteroids with propulsion systems that are limited in lower throttle range or maximum gimbal angle. Alternative strategies, such as on-off pulsing the thrusters to lower the effective thrust are considered. An example is described for deflecting asteroid 2008 EV5 (341843), which currently serves as the reference asteroid for the proposed Asteroid Redirect Robotic Mission.