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 \pm 0.0011 .
Orbital periods of P1 , P2 , and Charon are 38.2065 \pm 0.0014 , 24.8562 \pm 00013 , and 6.3872304 \pm 0.0000011 days , respectively .
The total system mass based on Charon ’ s orbit is 1.4570 \pm 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 \pm 0.0055 .
This new mass ratio implies a density of 1.66 \pm 0.06 g cm ^ { -3 } for Charon and 2.03 \pm 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 \pm 0.02 and color of ( B - V ) = 0.644 \pm 0.028 .
P2 has a mean opposition magnitude of V = 23.38 \pm 0.02 and color of ( B - V ) = 0.907 \pm 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 .