A low-thrust lunar cycler of the moons of Saturn

All our knowledge about Saturn and its icy ring system comes from the data obtained during the flybys of Pioneer 11, Voyager 1, Voyager 2 and Cassini, as well as from the observations carried out by Hubble Space Telescope. The discovery of water vapor plumes at the poles of Enceladus and other compelling evidence of the existence of subsurface water in the major moons of Saturn has driven scientific interest and revived plans to return to Saturn. In order to gain insight into the features of this planet and its Inner Large Moons (ILMs)--Mimas, Enceladus, Tethys and Dione--an in situ mission is needed. In general, orbit insertion at a giant planet is very demanding in terms of propellant, due to the large impulse required to achieve capture. It is even more challenging to achieve orbits around moons deep inside the planetary gravitational well, like the ILMs. The majority of the proposed solutions to tour the system of icy moons is based on the patched conics technique with impulsive maneuvers (i.e., chemical propulsion). The more efficient approach presented here is the concept of a lunar tour of the ILMs based on low-thrust (LT) propulsion and low-energy transfers in the circular restricted three-body problems (CR3BP) corresponding to Saturn and each moon. The hyperbolic invariant manifolds of planar Lyapunov orbits around the equilibrium points L1 and L2 of each Saturn-moon system are used to loop around the corresponding moon and to provide initial conditions to move between neighboring moons. These moon-to-moon transfers use a LT control law designed to minimize propellant consumption. LT, combined with a gravity assist with Jupiter, is also applied to reduce the hyperbolic excess speed at Saturn. This enables unpowered capture at Saturn by means of a Titan flyby. Results show that this mission concept saves a significant amount of propellant compared to the Cassini mission. Although LT yields longer transfer times than impulsive maneuvers, the spiraling transfers between moons can be exploited to collect data of the inter-moon environment, rings and moonlets.