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Orbits and Photometry of Pluto’s Satellites: Charon, S/2005 P1, and S/2005 P2

We present new astrometry of Pluto's three satellites from images taken of the Pluto system during 2002-2003 with the High Resolution Camera mode of the Advanced Camera for Surveys instrument on the Hubble Space Telescope. The observations were designed to produce an albedo map of Pluto, but they also contain images of Charon and the two recently discovered satellites S/2005 P1 and S/2005 P2. Orbits fitted to all three satellites are nearly coplanar and for Charon and P2 have eccentricities consistent with zero. The orbit of the outermost satellite, P1, has a significant eccentricity of 0.0052 ± 0.0011. Orbital periods of P1, P2, and Charon are 38.2065 ± 0.0014, 24.8562 ± 00013, and 6.3872304 ± 0.0000011 days, respectively. The total system mass based on Charon's orbit is × 1022 kg. We confirm previous results that orbital periods are close to the ratio of 6 : 4 : 1 (P1 : P2 : Charon), indicative of mean-motion resonances, but our results formally preclude precise integer period ratios. The orbits of P1 and P2, being about the barycenter rather than Pluto, enable us to measure the Charon-to-Pluto mass ratio as 0.1165 ± 0.0055. This new mass ratio implies a density of 1.65 ± 0.06 g cm-3 for Charon (603.6 km radius) and 2.03 ± 0.06 g cm-3 for Pluto (1153 km radius), thus adding confirmation that Charon is significantly less dense than Pluto. Finally, by stacking all images we can extract globally averaged photometry. P1 has a mean opposition magnitude of V = 24.39 ± 0.09 and a color of (B - V) = 0.64 ± 0.12. P2 has a mean opposition magnitude of V = 24.55 ± 0.10 and a color of (B - V) = 0.91 ± 0.15.