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 × 10 22 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.