Constrained control for soft landing on an asteroid with gravity model uncertainty

An autonomous control strategy is developed that enables soft landing of a spacecraft on a small rotating asteroid while exploiting a constant density ellipsoid gravity model. A disturbance canceling input observer is used to compensate for the error between the estimated and actual gravity force. An extended command governor is used to enforce constraints on control and landing speed while performing collision avoidance. The spacecraft landing mission is divided into two phases. In the first, circumnavigating phase an ellipsoidal constraint is placed around the asteroid that guarantees collision avoidance while bounding control. In the second, landing phase, the spacecraft trajectory is maintained within a pyramid that tapers near the landing point while spacecraft speed is reduced as it approaches the surface. Simulations performed on the full nonlinear model with actual gravity parameters demonstrate successful landing using the proposed controller scheme.