THEORY OF MOTION AND EPHEMERIDES OF HYPERION

We present here a new theory of motion for Hyperion, the seventh major satellite of Saturn. The Hyperion's motion is defined like in TASS 1. 6 for the other satellites (Vienne & Duriez 1995), by the osculating s aturnicentric orbital elements referred to the equatorial plane of Sat urn and to the node of this plane in the mean ecliptic for J2000.0. Th ese elements are expressed as semi-numerical trigonometric series in w hich the argument of each term is given as an integer combination of 7 natural fundamental arguments. These series collect all the perturbat ions caused by Titan on the orbital elements of Hyperion, whose amplit udes are larger than 1 km in the long-period terms and than 5 km in th e short-period ones. However, the convergence of these series is so sl ow that, in spite of several hundreds of terms, their internal accurac y over one century is about 200 km only. These series have been constr ucted in such a way that the fundamental arguments and the amplitude o f each term depend explicitly on 13 parameters (the twelve initial con ditions of the motions of Titan and Hyperion and the mass of Titan). T aking also account of the perturbations from other satellites and Sun, we have fitted these series to 8136 Earth-based observations of Hyper ion in the interval [1874-1985], giving a set of better values for the se parameters. In particular the mass of Titan is found equal to (237. 399 +/- 0.005) 10(-6) (in units of the Saturn's mass) and we discuss t his value in comparison with that [(236.638 +/- 0.008) 10(-6)] obtaine d by Campbell & Anderson from their analysis of the Voyager missions t o Saturn. The resulting fitted series allows us to produce new ephemer ides for Hyperion. Their comparison to those from Taylor (1992) shows that, with the same set of observations and the same way to weight the m, we obtain a root mean square (o-c) residual of 0''.156 while the ep hemerides of Taylor gives 0''203.