Low-thrust trajectory design and optimization of lunar south pole coverage missions

Abstract

A framework for designing and optimizing low-thrust trajectories for lunar south pole coverage missions is developed. Such missions may involve three, two, or even one satellite to maintain continuous communications between a lunar ground station and the Earth. Special emphasis is dedicated to single satellite communication links, which involve the design and discovery of novel pole-sitter orbits. Pole-sitters are possible, given the availability of an efficient low-thrust force in the model. Low-thrust acceleration can be delivered in various forms; solar sails and electric propulsion engines are obvious examples. Low-thrust propulsion may also be employed to construct transfer trajectories to the coverage orbits of interest as well as end-of-life transfers. Additionally, a low-thrust thruster allows a spacecraft to shift between lunar coverage orbits. In this scenario, an optimal control-based approach is applicable for rapidly computing trajectories, however, in general, the many complexities involved in generating the trajectories are best solved with a direct transcription approach using collocation and mesh refinement. This general process is robust and allows for the inclusion of an unknown control history, path constraints, and the simultaneous optimization of multiple phases while exploiting matrix sparsity for maximum computational efficiency. Even incorporating higher-fidelity dynamical effects, the pole-sitter solutions can be sustained as a long-duration option, using a solar sail, or as a temporary option in excess of one year on a small 500 kg spacecraft, using a solar electric propulsion engine comparable to existing technology