Dynamical Characterization and Stabilization of Large Gravity-Tractor Designs

Abstract

This paper addresses the problem of stabilizing and regulating, to a desired reference, the attitude and position of large gravity-tractor spacecraft relative to near-Earth objects to be deflected from Earth impact. We analyze gravity tractors constructed in both the previously proposed pendulum configuration and a novel bar configuration. These interact with the deflection target through mutual gravitational potential, such that the system has fully coupled rotational and translational dynamics. The target body is represented by increasingly more realistic models (sphere, oblate spheroid, general triaxial ellipsoid, and polyhedral mesh). For the simplest of these, an eigenstructure-based controller is detailed, followed by an energy-based controller for the more general target representations, with additional dissipative feedback used in each case. It is demonstrated with theory and simulation results that both the pendulum and novel bar gravity tractors are suitably controlled to the most desirable precessing downtrack-towing-direction vector using these controller designs. They are robust in the presence of realistic perturbations and the effects of full geometric detail for both the natural and artificial bodies. Further, this paper introduces important performance metrics for gravity-tractor system design, which quantify the difficulty of control for each case. Based on this, some prior claims made to the effect that because the gravity tractor does not need to physically connect with the asteroid, there is no need for prior knowledge of the asteroid's shape, composition, rotation rate, etc., appear to be incorrect.