  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 nearly 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)
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/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 color of (B-V) =
0.64 +/- 0.12. P2 has a mean opposition magnitude of V=24.55 +/- 0.10 and color
of (B-V) = 0.91 +/- 0.15.
