Robust Spin Control Design for the AOSAT+ Mission Concept

The surfaces of asteroids are a challenging environment to explore due to their low gravity. Active small-body missions rely on short-duration touch-and-go operations to mitigate this risk. An in-depth understanding of the surface geophysics of asteroids and comets can open the door to prolonged surface and subsurface exploration of these small bodies. We propose the AOSAT+ mission concept, which will provide rich physics data of a simulated asteroid surface. The mission consists of a 12U CubeSat that will operate as a centrifuge laboratory in low Earth orbit (LEO). The CubeSat will carry 2.5 kg of crushed Allende meteorite, along with a suite of science instruments. The spacecraft will rotate at 0.1-1.1 RPM to simulate the milli-gravity environment of a desired small body. A major challenge with operating a centrifuging spacecraft is that it contains shifting masses, which result in perturbation torques on the spacecraft. This requires a robust attitude controller to spin the spacecraft at its target rotation speed. This article presents the development of a sliding-mode attitude control law that enables the operation of the AOSAT+ centrifuge mode. The perturbations of the regolith are modeled using a discrete element model (DEM), where the regolith grains are treated as inelastically colliding hard spheres. We begin by presenting a detailed overview of the AOSAT+ mission concept and its different operations. The regolith motion model implementation and the detailed derivation of the required sliding-mode controller are then presented. The constraints presented by the actuators and tools to study their limitations are then developed. Finally, the controller is shown to successfully demonstrate the spin rate requirements of the AOSAT+ centrifuge mode. Key insights on the operation of the centrifuge mode and important mission design considerations on the spacecraft are then noted.