Numerical Analysis of Polar Orbits for Future Enceladus Missions

Abstract

Enceladus is one of the few objects in our Solar System that probably harbors a habitable environment. This makes it a highly interesting target for planetary exploration and the European Space Agency (ESA) has decided to send its large mission (L4) to Enceladus. With the active regions located at the South pole of the moon a polar orbit is most desirable for revealing the mechanism that powers the jets and to perform a chemical analysis of the material ejected from the deep interior of Enceladus. We carried out a comprehensive numerical integrations of spacecraft orbits, with the aim to find suitable candidate orbits for a future mission to Enceladus. All the relevant perturbations caused by mainly Saturn, as well as the Sun, Jupiter, and the other moons of the Saturn system, and also solar radiation pressure, are taken into account. We have considered the higher degree and order Stokes coefficients of Enceladus’ and Saturn’s gravity fields provided in Park et al., 2024. Furthermore, we performed a grid search to identify suitable orbits in inertial space by varying orbital parameters such as semi-major axis (330 to 420 km), inclination (40° to 120°) and longitude of ascending node. Moderately inclined orbits (inclination between 45° and 60°) covering the equatorial and mid-latitude regions of Enceladus were found to be stable from several months up to years. In contrast, the more useful polar mapping orbits were found to be extremely unstable due to the so-called “Kozai mechanism”, which causes the spacecraft to impact the moon’s surface within a few days. However, an example of a highly inclined orbit was found with inclination ofapproximately 76°, which had an orbital life time of 13 days. A longer missionduration in this orbit would require correction maneuvers every few days. This would provide coverage of the tiger stripes region and allow for a near-global characterization of the surface. We also determined the delta-v that would be necessary to maintain such an orbit over a mission duration of several months.