Low-thrust trajectory design for icy moons orbiters using multi-body techniques

Icy moons of our Solar system are currently one of the focuses of the leading space agencies in the search for extraterrestrial life. Especially, Enceladus is a prime target because of the presence of geyser-like jets of water emanating from its south pole. Future missions to Enceladus would benefit from inter-moon transfers of the Saturnian system to reach Enceladus. However, such trajectories are challenging to design, due to the highly nonlinear, often chaotic, multi-body dynamics with close encounters of the planetary moons. The trajectory design is even more complex when low-thrust propulsion is used due to the long-duration, multi-revolution nature of the solutions. The approach outlined in this paper confronts this shortcoming by exploiting multi-body dynamics to construct a ‘resonant hopping’ trajectory, which is then optimized using indirect methods. Unstable resonant orbits are pre-computed and provide a starting point to a multiple indirect shooting method in a forward–backward fashion. The insights from multi-body dynamics steer the algorithm to a near-ballistic solution. The use of indirect optimization allows the design of long-duration, multi-revolution low-thrust trajectory, with a limited number of optimization variables. Finally, forward–backward shooting technique reduces the sensitivities to constraints satisfaction. This strategy is employed to design a low-thrust inter-moon transfer between a close resonant orbit of Tethys and a near rectilinear halo orbit around Enceladus.