Exploring more solutions for low-energy transfers to lunar distant retrograde orbits

This study aims to explore more solutions for low-energy transfers to lunar distant retrograde orbits (DROs) from the vicinity of the Earth. Millions of transfer trajectories from a lunar free-return orbit (LFO) to a prescribed DRO were computed using multiple powered lunar flybys (PLFs) and weak-stability-boundary (WSB)-like ballistic transfer. We proposed a two-step design method, consisting of a database creation and trajectory patching, to construct low-energy LFO–DRO transfers in planar bicircular restricted four-body dynamics with the Sun, Earth, and Moon as primary bodies. The parallel computation technique allows the computation of millions of solutions with times of flight (TOFs) up to 135 days and a total velocity impulse (Δ V) of no more than 350 m/s, although this design method requires substantial computational load. These solutions help us identify key flight information, such as the Δ V–TOF Pareto fronts and launch windows for rendezvous with a station in a DRO. Low-energy transfers to a DRO can be achieved by exploiting single or multiple PLFs and WSB-like ballistic arcs at the expense of elongated TOFs. Moreover, triple PLFs render many more options for the spacecraft to accomplish the rendezvous of DROs. The WSB-like ballistic arcs to a DRO in this study exhibit new features compared with conventional and traditional WSB concepts.