The determination of Iapetus mass from Doppler tracking of the Cassini spacecraft

On October and December 2004 the Cassini spacecraft performed two flybys of Saturn’s moon Iapetus. Although the closest approach distance was large and the encounters barely deserve the name of flybys, they provided the first opportunity for an accurate determination of the satellite’s mass and density. The good quality of the Cassini X-band Doppler data allowed a substantial reduction of the experimental uncertainty obtained with past measurements. The orbital fit of the spacecraft trajectory was carried out over arcs of 15 days about the closest approach and used a limited set of solve-for parameters. Our best estimate of the satellite’s gravitational parameter results from the first and more distant flyby. The second flyby, occurring at a much closer distance, gave a smaller formal error and a rather consistent estimate, but the orbital solution was much more affected by the uncertainties in Iapetus ephemerides and likely to be biased.