J2-perturbed low-energy orbits around Enceladus

The discoveries made by Cassini of geyser-like jets of vapour and organic compounds at the southern polar region of Enceladus have given impulse to a detailed study of this moon. As a result, a number of mission plans for the in-situ robotic exploration of Enceladus have been proposed by scientific communities and leading space agencies. The mission objectives of those plans can only be accomplished with orbits that provide extended observations of the southern polar surface of Enceladus. In a previous contribution, heteroclinic connections between halo orbits around the collinear equilibrium points L1 and L2 of the unperturbed Saturn-Enceladus circular restricted three-body problem have been proposed for the purpose. Due to the low altitude of these orbits with respect to the surface of Enceladus and the perturbations of the gravity field of Saturn, the effect of the second zonal harmonics of the two bodies on these low-energy solutions need to be assessed. The present contribution refines the previously computed low-energy trajectories in the J2-perturbed circular restricted three-body problem in which the primaries are Saturn and Enceladus. Halo orbits and their stable and unstable hyperbolic invariant manifolds are obtained in this new framework and used to construct heteroclinic connections in the enhanced dynamical model. Maneuver-free trajectories are obtained and compared with their unperturbed counterparts. Eventually, the performance of these solutions as science orbits is assessed by evaluating their speed, lunar surface coverage, time of flight and height above the lunar surface over the transfers. The results show a good agreement with the solutions obtained with the unperturbed model, suggesting that these trajectories can serve the purpose of observing Enceladus and the unperturbed model is a valid tool for a mission preliminary analysis. Furthermore, the second zonal harmonic term of Saturn is found to have a larger effect than the oblateness of Enceladus.