Estimation and Modeling of Enceladus Plume Jet Density Using Cassini Flight Data

After an interplanetary cruise of almost seven years, the Cassini spacecraft arrived at Saturn on 30 June 2004. In 2005, Cassini completed three flybys of Enceladus, a small icy satellite of Saturn. Observations made during these flybys confirmed the existence of water vapor plumes in the south polar region of Enceladus. During an Enceladus flyby, plume jets imparted on the spacecraft resulted in small but visible attitude control errors. Using the known and unique transfer function between the disturbance torque and the attitude control error, the disturbance torque could be estimated. Furthermore, given good estimates of the spacecraft’s projected area, center of pressure location, and spacecraft velocity, the time history of the Enceladus plume density could be reconstructed. Next, the density due to each plume jet as a function of both the radial distance of the spacecraft from the plume source and the angular distance of the spacecraft from the axis of symmetry of the jet was modeled. By comparing the reconstructed plume density with that predicted by the plume model, values of the plume model parameters are determined. The validity of this plume density model is confirmed using flight data obtained from four low-altitude flybys of Enceladus in 2008–2011.