Enceladus Mission Architecture Using Titan Aerogravity Assist for Orbital Capture About Saturn

R ECENTobservations of the Saturn system have generated great interest in the moon Enceladus. Water geysers have been identified in the southern hemisphere and have been found to contain possible organic compounds [1]. These observations strongly support a return to the Saturn system specifically designed to evaluate Enceladus in greater detail and perhaps to return a sample from the geyser’s plume. We previously described an aerogravity assist (AGA) maneuver, in which Titan’s atmosphere and gravitational field are used to decelerate a spacecraft and deflect its trajectory, resulting in a closed orbit about Saturn [2]. A similar concept using wave-rider configurations was described by Randolph and McRonald [3,4] and evaluated using an analytical approach. In contrast, we have numerically simulated AGA trajectories for low and medium lift-todrag (L=D) vehicles. In this paper, we describe Titan AGA maneuvers in greater depth, and evaluate their potential for orbital capture of a Cassini-class vehicle. The targeted Saturn orbit has a periapsis near the radius of Enceladus’s orbit, allowing repeated close passes. This orbit can be achieved by directing the outbound hyperbolic excess speed (V1) with respect to Titan nearly opposite in direction to Titan’s orbital velocity, resulting in a final apoapsis near Titan’s orbital radius. Depending on the interplanetary trajectory and the exact Titan encounter location, the proposed maneuver would provide a velocity increment ( V) in excess of 6 km=s. Titan has a near-circular, equatorial orbit about Saturn at a radius from the planetary center of 1:22 10 km. Its orbital velocity (VT) and period are 5:58 km=s and 15.95 Earth days, respectively. Titan’s orbit lies well outside the ring system, which extends in the equatorial plane to a radius of approximately 480,000 km. Enceladus orbits in nearly the same plane, at a radius of 238,000 km andwith an orbital period of 32.9 h. The mission concept is illustrated in Fig. 1, where the upper diagram shows Titan’s velocity vector and that of the probe before and after the AGA maneuver, and the lower panel illustrates the range of potential directions for the outbound V1, any of which will produce a periapsis at Enceladus’s orbital radius. To achieve this desired periapsis, the probe’s velocity immediately after completion of the Titan AGA should have a magnitude 3:18 km=s with respect to (WRT) Saturn and a flight path angle of 0 deg. The various possible directions for this velocity vector (Fig. 1b) determine the inclination of the final orbit about Saturn. Some variation in the velocity after the AGA from this target value may be acceptable (or even desirable) and would yield a periapsis slightly inside or outside Enceladus’s orbital radius. In addition, the vehicle may have a radial velocity component WRT Saturn after the maneuver, resulting in an apoapsis outside Titan’s orbital radius. For simplicity, the latter scenario is not considered in this preliminary study. Titan is particularly well suited for an AGA maneuver because of its relatively weak gravitational field, coupled with its unique atmospheric structure. Although Titan’s radius is only 2575 km, atmospheric pressure at ground level is approximately 1.6 times that on Earth, and the atmosphere extends to an altitude of approximately 1000 km (compared with approximately 125 km for Earth). These factors combine to yield relatively low atmospheric flight velocities at a given density level for a planetary entry probe, especially when compared with a vehicle entering Saturn’s atmosphere. This makes a Titan AGAmaneuver preferable to direct aerocapture using Saturn’s atmosphere because of the much lower entry speed and the lower aerodynamic heating. The less-severe heating translates directly into weight savings in the vehicle’s thermal protection system (TPS).