Masses of Nix and Hydra

A four-body orbit solution for the Pluto system yields GM values of 870.3 ± 3.7, 101.4 ± 2.8, 0.039 ± 0.034, and 0.021 ± 0.042 km3 s−2 for Pluto, Charon, Nix, and Hydra, respectively. Assuming a Charon-like density of 1.63 gm cm−3, the implied diameters for Nix and Hydra are 88 and 72 km, leading to visual geometric albedos of 0.08 and 0.18, respectively, though with considerable uncertainty. The eccentricity of Charon’s orbit has a significant nonzero value; however, the 0.030 ± 0.009 deg yr−1 rate at which the line of apsides precesses is insufficient to explain the difference in the longitude of periapsis seen in the orbits fitted to the 1992–1993 and 2002–2003 data sets. The mean orbital periods for Hydra, Nix, and Charon are in the ratios of 6.064 ± 0.006 : 3.991 ± 0.007 : 1, but we have not identified any resonant arguments that would indicate the existence of a mean motion resonance between any pairs of satellites.