S/2004 S 5: A new co-orbital companion for Dione

We describe the astrometry and dynamics of S/2004 S 5 (provisionally named Polydeuces), a new satellite of Saturn, discovered in images recorded using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. A total of 119 observations are presented, 101 of which were obtained with Cassini's narrow-angle camera and 18 using the wide-angle camera. Orbit determinations, based on a uniformly precessing ellipse model clearly showed S/2004 S 5 to be a possible co-orbital of Dione. By fitting a numerical solution of the full equations of motion to the available observations, we show that S/2004 S 5 is executing tadpole motion around Dione's L5 Lagrangian point, with a libration amplitude varying between +21.16±0.03° and −31.68±0.06° over a period of 791.3±0.1 days.