+ Control of Spacecraft In Proximity to Asteroids

Control algorithms based on extended command governors and model predictive control are being developed for operating autonomous spacecraft in proximity to an asteroid, including autonomous landing. Figure 1 shows the implementation of an MPC controller in simulations for spacecraft autonomous asteroid landing. The spacecraft flies around an asteroid without collision and approaches the landing target while remaining inside the prescribed landing cone near the end of the motion subject to the constraints on thrust and despite the uncertain gravity field.

The extended command governor is used to prescribe the way point target to the inner loop spacecraft controller, the rotating hyperplane constraint is used to separate the asteroid from the spacecraft during fly around maneuver and avoid collision with the surface, a landing cone is used as a constraint during landing, and input observer based compensation is employed for gravity field uncertainty compensation. See reference [1] for details.

Figure 1: Fly around and landing on an asteroid Eros controlled by an Extended Command Governor. See reference [1] for details.

The design of Model Predictive Controllers (MPC) is addressed in reference [2], where also the comparison with the solution based on ECG is made. See Figure 2. Figure 3 illustrates the ability of the spacecraft to achieve successful landing with NMPC controller despite the uncertainty in the initial spacecraft position.

Figure 2: Fly around and landing on an asteroid Eros controlled by different Model Predictive Controllers and comparison with ECG. See reference [2] for details.

Figure 3: Closed-loop trajectories from different initial spacecraft positions generated by Nonlinear MPC controller for the autonomous landing on Eros. See reference [2] for details.

References:

[1] Dunham, W., Petersen, C., and Kolmanovsky, I.V., “Constrained control for soft landing on an asteroid with gravity model uncertainty,” Proceedings of 2016 American Control Conference (ACC), Boston, MA, 2016, pp. 5842-5847, doi: 10.1109/ACC.2016.7526585

[2] Liao-McPherson, D., Dunham, W., and Kolmanovsky, I.V., “Model Predictive Control strategies for constrained soft landing on an asteroid,” Proceedings of AIAA/AAS Astrodynamics Specialist Conference, Long Beach, California, AIAA Paper 2016-5507, doi: 10.2514/6.2016-5507, 2016.