Trajectory Optimization for Asteroid Landing Considering Gravitational Orbit-Attitude Coupling

This paper proposes a trajectory optimization method considering gravitational orbit-attitude coupling (GOAC) for asteroid landing. First, by modeling the spacecraft as a rigid body rather than a point mass, and using the polyhedral method to describe the irregular gravitational field of the asteroid, GOAC is fully embedded into the 6 degree-of-freedom (DOF) dynamic model of the spacecraft. Second, attitude control, which is assumed to consume electric energy, is introduced into the optimization, the indirect method is adopted to formulate the 6-DOF fuel-optimal control problem, and the boundary conditions are modified to expand the search space of the problem. Third, a two-phase homotopic approach is proposed to realize a smooth continuation from the 6-DOF energy-optimal problem to the fuel-optimal problem. Finally, a numerical example is presented to demonstrate the feasibility of the proposed method, and the comparison results show that the obtained 6-DOF optimal trajectory can guarantee the fuel-optimality of the landing mission under GOAC.