  We present a new orbit and mass solution for the four small satellites of
Pluto: Styx, Nix, Kerberos, and Hydra. We have reanalyzed all available
observations of the Pluto system obtained by the Hubble Space Telescope (HST)
from 2005 to 2019 with the ACS, WFPC2, and WFC3 instruments, as well as the New
Horizons LORRI images taken on approach to Pluto in 2015. We have used this
high precision astrometry to produce updated orbits and mass estimates with
uncertainties for all four of the small satellites. We find that the masses of
Nix and Hydra are smaller than previously published estimates, with a dynamical
mass of 1.8$\pm$0.4$\times$10$^{-3}$ km$^3$/s$^2$ (2.7$\pm$0.6$\times$10$^{16}$
kg) for Nix and 2.0$\pm$0.2$\times$10$^{-3}$ km$^3$/s$^2$
(3.0$\pm$0.3$\times$10$^{16}$ kg) for Hydra. These masses are 60% and 63% of
the mean estimates by Brozovic et al. (2015), although still consistent with
their 1-sigma uncertainties, and correspond to densities of 1.0$\pm$0.2
g/cm$^3$ for Nix and 1.2$\pm$0.2 g/cm$^3$ for Hydra given the moon volume
estimates from Porter et al (2021). Although these densities are consistent
with a range of ice-rock compositions, depending on the unknown bulk porosity
in the moon interiors, the moons' high albedos and predominantly icy surfaces
are most easily explained if their interiors are ice-rich. The tiny masses of
Kerberos and Sytx remain very poorly constrained; we find 1-$\sigma$ upper
limits for the dynamical mass of Styx to be 3$\times$10$^{-5}$ km$^3$/s$^2$
(5$\times$10$^{14}$ kg) and for Kerberos 5$\times$10$^{-5}$ km$^3$/s$^2$
(8$\times$10$^{14}$ kg), consistent with densities of $<$2.1 g/cm$^3$ for both
bodies.
