Adaptive Terminal Sliding Mode Control for Asteroid Hovering by Solar Sailing: Application to 433 Eros

In this paper, we propose a second-order sliding mode control law of solar sail to hover on displaced orbits above an asteroid. To overcome the difficulties of solar sail control, firstly, dynamics in cylindrical coordinates are used and only the hovering radius and height are controlled, neglecting the polar angle; secondly, the angular velocity of two attitude angles is taken as the control input instead of the angles themselves; lastly, an adaptive estimation law is applied to increase the robustness to gravity uncertainty. The case of hovering on displaced orbits above 433 Eros is simulated. The effect of different hovering radius, height and sunlight incidence direction are studied. In addition, the robustness of the control law is tested against unknown gravity disturbances and imprecise sail force model. This work successfully demonstrates that it is feasible to achieve an asteroid-hovering mission using an underactuated solar sail with only two controllable attitude angles