Hi All

New arXiv.org abstract... Pluto and Moons

...which refines a lot of the ambiguities. In sum...

Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2
Authors: M. W. Buie (1), W. M. Grundy (1), E. F. Young (2), L. A. Young (2), S. A. Stern (2) ((1) Lowell Observatory, (2) Southwest Research Institute)
Comments: 21 pages, 5 figures, 4 tables

We present new astrometry of Pluto's three satellites from images taken of the Pluto system during 2002-3 with the High Resolution Camera (HRC) mode of the Advanced Camera for Surveys (ACS) 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 co-planar 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 1.4570 +/- 0.0009 x 10^22 kg. We confirm previous results that orbital periods are close to the ratio of 6:4:1 (P1:P2:Charon) indiciative 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/Pluto mass ratio as 0.1165 +/- 0.0055. This new mass ratio implies a density of 1.66 +/- 0.06 g cm^-3 for Charon and 2.03 +/- 0.06 g cm^-3 for Pluto thus adding confirmation that Charon is somewhat under-dense relative to Pluto. Finally, by stacking all images, we can extract globally averaged photometry. P1 has a mean opposition magnitude of V=24.39 +/- 0.02 and color of (B-V) = 0.644 +/- 0.028. P2 has a mean opposition magnitude of V=23.38 +/- 0.02 and color of (B-V) = 0.907 +/- 0.031. The colors indicate that P1 is spectrally neutral and P2 is slightly more red than Pluto. The variation in surface color with radial distance from Pluto is quite striking (red, neutral, red, neutral) and begs further study.

...notice the new mass and density constraints and the odd colour scheme.

Adam