Close proximity asteroid operations using sliding control modes

Due to their uncertain dynamical environment, close proximity operations around small celestial bodies are extremely challenging. In this paper, we show that the Multiple Sliding Surface Guidance (MSSG) algorithm, already proposed for autonomous asteroid pin-point guidance, can be extended to guide the transition of the spacecraft from any two desired states, including hovering, surface and orbital states. MSSG is based on Higher Order Sliding Mode (HOSM) control theory and takes advantage of the fact that the motion of the spacecraft around asteroids exists in a 2-sliding mode, i.e. The acceleration command appears at the second derivative of the defined sliding surface. The proposed algorithm is constructed by the proper concatenation of two sliding surfaces and takes advantage of the system's ability to reach the sliding surfaces in finite time. Importantly, the MSSG algorithm does not require either ground-based or on-board trajectory generation but computes an acceleration command that targets a specified state based on purely knowledge of the current and desired position and velocity. The classes of trajectories generated in this fashion are a function of the current and final states as well as of the guidance gains. Moreover, the controller is shown to be globally stable in the Lyapunov sense. MSSG is implemented in simulation scenarios comprising a variety of operations around a model asteroid, demonstrating the ability of the algorithm to guide the system between 1) two hovering states, 2) surface and hovering states and 3) surface to hovering. The MSSG algorithm is also shown to be able to shape the closed-loop trajectories to satisfy the requirements imposed by the need to execute a defined set of close-proximity operations.