Titan Density Reconstruction Using Radiometric and Cassini Attitude Control Flight Data

Abstract

This paper compares three different methods of Titan atmospheric density reconstruction for the Titan 87 Cassini flyby. T87 was a unique flyby that provided independent Doppler radiometric measurements on the ground throughout the flyby including at Titan closest approach. At the same time, the onboard accelerometer provided an independent estimate of atmospheric drag force and density during the flyby. These results are compared with the normal method of reconstructing atmospheric density using thruster on-time and angular momentum accumulation. Differences between the estimates are analyzed and a possible explanation for the differences is evaluated. Nomenclature F/D Slope = On-board estimate of peak external torque FSDS = Flight Software Dynamic Simulation HGA = High Gain Antenna INMS = Ion and Neutral Mass Spectrometer RCS = Reaction Control System RWA = Reaction Wheel Assembly TCA = Titan Closest Approach I. Introduction-Cassini Mission to Saturn and Titan A. Overview of Cassini Mission The Cassini-Huygens mission is a collaborative effort between the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Italian Space Agency (ASI) to explore the Saturnian system. Launched in October 1997 onboard the powerful Titan IVB/Centaur, the Cassini-Huygens spacecraft still required multiple gravity assist flybys of Venus, Earth, and Jupiter to achieve the necessary velocity to reach Saturn. On June 30, 2004, Cassini achieved orbital capture at Saturn by firing its main engine for 85 minutes to slow its velocity by about 626 m/s. About 5 months later, the 320 kg ESA-built Huygens probe was successfully deployed from Cassini, and descended into Titan's atmosphere. Following playback of the probe's collected atmospheric data to Earth, Cassini then continued its primary mission of studying the Saturnian system via its suite of 12 remote and direct sensing science instruments. B. Saturn's Moon Titan Titan is an intriguing world larger than our own moon with a dense atmosphere and liquid hydrocarbon lakes on its surface. Because Titan is less massive than Earth, its gravity does not hold on to its gaseous atmosphere as tightly as Earth does, so its atmosphere extends roughly ten times farther into space than Earth's atmosphere. At the surface of Titan the atmospheric pressure is approximately 1.5 times greater than that of Earth at sea level with atmospheric density a factor of 4 larger. However, the chemical composition of Titan's atmosphere differs from Earth's; it is