Post-new-horizons Orbits and Masses for the Satellites of Pluto

Abstract

We report on ephemerides for the satellites of Pluto based on the large set of astrometric measurements. Our orbit fit yielded the following masses with 1 σ uncertainties: GM Pluto = 869.3 ± 0.4 km 3 s −2 , GM Charon = 106.1 ± 0.3 km 3 s −2 , GM Nix = 1.50 ± 0.52 × 10 −3 km 3 s −2 , GM Hydra = 2.01 ± 0.27 × 10 −3 km 3 s −2 , corresponding to the densities of ρ Pluto = 1.853 ± 0.004 g cm −3 , ρ Charon = 1.705 ± 0.006 g cm −3 , ρ Nix = 0.88 ± 0.31 g cm −3 , and ρ Hydra = 1.21 ± 0.19 g cm −3 . Masses of Kerberos and Styx remain unconstrained, and it is unlikely that we will be able to measure them even if we obtain abundant 1 mas precision astrometry during the next 20 yr. We summarize the results of orbit integration in terms of osculating and precessing ellipse model mean elements. All satellites reside in near-circular orbits, and Kerberos and Styx have 0.4 deg and 0.3 deg inclinations with respect to Charon’s orbit plane. The nodal regression periods for Kerberos and Hydra are ∼9 and ∼14 yr respectively. We found that Charon’s orbit pole can be approximated as: R.A. = 133.0073 + 0.0036 T deg, and decl. = –6.2449 + 2.5 × 10 −5 T deg, where T is Julian centuries from the epoch J2000, based on 5000 yr of orbit integration.