Dynamical Characterization, Control, and Performance Analysis of Gravity Tractor Operation at Binary Asteroids
Fahnestock, Eugene
University of Michigan / NASA Jet Propulsion Laboratory

The so-called "gravity tractor" (GT), proposed by Lu et al [1] as a method for altering the velocity of a Near Earth Object (NEO) to prevent it from impacting Earth, has been the subject of several recent studies and analyses [2, 3, 4, 5]. The spacecraft involved may be large or small in mass and/or extent, and hovers in close proximity to the NEO at constant or slowly rotating relative position and attitude with respect to the NEO. A low thrust, high-Isp propulsion system (electric propulsion, solar sails, etc.) continuously operates to balance gravitational forces exerted on the spacecraft by the NEO. If all thruster exhaust avoids impinging on NEO surfaces, the net result is a continuous acceleration of the combined NEO and spacecraft system. In all studies to date, the NEO has been treated as a single body, despite much evidence indicating high probability (≈15±4%) of multiple-component systems among small NEOs [6, 7, 8, 9]. In this paper, we attempt to extend to the case of a binary NEO system our prior work in [2] for developing and demonstrating two control laws (one eigenstructure-based, one energy-based) for maintaining the desired artificial equilibrium relative to a deflection target. We find only the energy-based method, involving addition of an artificially defined potential field with appropriate velocity damping, is feasible once both component bodies are present. We develop a proof of Lyapunov stability of the energy-based controller for the binary case, and present a few detailed numerical simulations under the action of the controller to further validate operation with it. Next we present a study of geometric restrictions on GT thruster cant angle specific to a binary NEO, over the parameter space of hovering distance and inclination of the binary system with respect to the desired towing direction. We also compute several important performance metrics and system efficiencies over the same parameter space, assuming a static setup maintaining exactly the desired artificial equilibrium. Finally, we study a few carefully selected points in this parameter space in detail by the same methodology used in [5], but instead employing the controller developed herein. This yields a comparison of the performance metrics as calculated for full-detail dynamics operation against their corresponding static values.

References
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