Advanced modeling of optimal low-thrust lunar pole-sitter trajectories

One immediate and cost-effective solution for lunar south pole coverage is a low-thrust pole-sitter spacecraft. When the spacecraft's fuel is nearly expended, it can also be used as part of a larger constellation that might require more time to deploy. This study examines the feasibility of the lunar pole-sitter from a dynamical standpoint. The model includes the effects of Earth oblateness, solar wind, gravity perturbations, and shadowing on a 500 kg spacecraft equipped with an NSTAR thruster. Lunar librations are also incorporated into path constraints on elevation angle and altitude from a potential ground station at the Shackleton crater. The solutions utilize multiple phases, including transfer-out from the International Space Station and spiral-down into a stable lunar orbit once the pole-sitting period is completed. Computation of the trajectory is based on a 12th-order collocation scheme. A direct transcription routine maximizes the time in the coverage phase, increasing the time span from 387 to 451 days. During this period, the minimum elevation angle relative to the Shackleton crater site is 6°.