Attitude Control of the Asteroid Redirect Robotic Mission Spacecraft with a Captured Boulder

NASA's Asteroid Redirect Robotic Mission (ARRM) aims to pick up a boulder from of a large asteroid and transport it to a distant retrograde orbit around the Moon for future exploration by a manned mission. In this paper, we present a detailed analysis for one of the main control challenges in ARRM, i.e., three-axis attitude control of the ARRM spacecraft with the captured boulder in the presence of large uncertainties in the physical model of the boulder. We first present a 30 degree-of-freedom nonlinear dynamic model of the ARRM spacecraft and boulder combination. We then linearize this nonlinear model about the nominal operating conditions to study the system's modal properties. A finite element model of the ARRM spacecraft and boulder combination is used to validate our model. We then present linear and nonlinear control laws for the attitude control problem. Both the proportional-derivate based linear controller with lead-lag compensator and roll-off filter and the robust nonlinear tracking control law that tracks a derivative plus proportional-derivate based desired attitude trajectory give robust performance over the range of boulder parameters. We present a detailed comparison of these control laws and also present some design guidelines for the ARRM spacecraft.