Investigation of Titan Aerogravity Assist for Capture into Orbit About Saturn

Abstract

Introduction A EROCAPTURE has been studied extensively for application on manned Mars missions,1,2 and, more recently, for robotic probes to Neptune and Titan.3,4 However, use of traditional aerocapture at the gas giants has limited appeal because of very high atmospheric entry speeds, severe aerothermal heating, and heavy ablative heat shields.5 The present study considers the application of a related maneuver known as an aerogravity assist (AGA), in which the atmosphere and gravitational field of Titan would be used to decelerate a spacecraft and deflect its trajectory, resulting in capture into a closed orbit about Saturn. Because of its high atmospheric density, Titan is well suited for such a mission design. This approach is more appealing than capture using the atmosphere of Saturn itself, because of the much lower entry speed at Titan and the lower heating rate and heat load. The less severe aerothermal environment translates directly into weight savings in the vehicle’s thermal protection system. Titan has a near-circular, equatorial orbit about Saturn at a radius from the planetary center of 1.22 × 106 km and an orbital velocity (VT ) of approximately 5.57 km/s. This orbit is well outside the ring system, which extends in the equatorial plane to a radius of approximately 480,000 km. A wide range of potential target orbits about Saturn could be achieved by means of a successful AGA at Titan. The final Saturnian orbit will depend on both the orientation and magnitude of the probe’s outbound hyperbolic excess speed with respect to Titan (V T ∞) after the AGA. By lining up the outbound V T ∞ with VT , an orbit can be reached with a periapsis radius equal to that of Titan’s orbit and a higher apoapsis. If the outbound V T ∞ is in the opposite direction, the final orbit will have an apoapsis radius equal to that of Titan’s orbit and a lower periapsis radius (Fig. 1). The proposed strategy could be used on a Cassini-type mission with a Saturnian orbital periapsis of approximately 160,000 km, allowing the probe to pass through the gap between rings F and G. Achieving this periapsis radius will require an outbound V T ∞ of approximately 2.89 km/s, opposite in direction to Titan’s orbital