From hyperbolic arrival to a lunar tour of the Inner Large Moons of Saturn

Since the discovery of geyser-like jets of water vapor and ice particles from an ocean beneath its icy crust, Enceladus has turned into a promising lead in our search for habitable worlds. Following the observations of the Cassini spacecraft, Mimas, Tethys and Dione have also gained the attention of the scientific community with questions regarding their age, formation and habitability. The renewed interest in the system of Saturn has led to the preparation of numerous plans for follow-up missions focusing on the in-situ exploration of these moons. In a recently proposed lunar tour of the Inner Large Moons (ILMs, i.e., Mimas, Enceladus, Tethys, Dione), the scientific observations of individual bodies are carried out from low-energy orbits computed in the circular restricted three-body problems (CR3BPs) where Saturn and each moon act as primaries. Such orbits are heteroclinic and homoclinic connections of Lyapunov orbits around the equilibrium points L1 and L2 of each system, whereas the connecting legs of the tour (i.e., the trajectories that depart a moon and approach another moon in the system) are two-body, low-thrust transfers. The present investigation addresses the design of the trajectory that connects the arrival at Saturn from interplanetary trajectory with the tour. The design of the interplanetary transfer, which was the subject of a previous contribution, relies on a combination of planetary gravity assists and low-thrust propulsion to reduce (to 1 km/s) the hyperbolic excess velocity of the spacecraft at Saturn. Here, we propose a sequence of gravity assists with Titan, Rhea and the ILMs themselves to further decrease the energy of the spacecraft and approach the scientific targets with a suitable relative speed to initiate the tour, thus minimizing the use of fuel. In this way, we propose a mission scenario in which the spacecraft departs Earth, reaches Saturn, approaches the ILMs and carries out a complete observation tour of these objects.