Diving deeper: Exploring the feasibility of lowering Cassini's final orbits

During the final five orbits of Cassini's mission, the spacecraft will get closer to Saturn than it has ever been. These five orbits were designed to be as deep in the atmosphere as Cassini could safely fly; however, recent occultation data of Saturn's atmosphere suggest that it is contracting. Given this contraction, the primary concern during these orbits has shifted from spacecraft health and safety to loss of science value. This paper explores a scenario for modifying the Cassini spacecraft's trajectory, during these final orbits, such that it dips deeper into Saturn's atmosphere. This scenario describes the method for in-situ detection of Saturn's atmospheric state, the locations and sizes of maneuvers that would reduce the final periapsis altitudes, the effects of such maneuvers on the remaining trajectory, and the risks involved. The result is that a periapsis-lowering, “pop-down” maneuver is feasible during Cassini's final orbits. Risk to the spacecraft is minimized by using the attitude control thrusters as density detectors during the first three atmospheric transits of the final five orbits. Should these transits reveal sufficiently low density and should sufficient propellant remain, then the Cassini project will consider performing the maneuver.