New Horizons and Hubble Space Telescope Photometry of Nix and Hydra

New Horizons has provided the first spatially-resolved images of Pluto’s small satellites, enabling the characterization of albedo and scattering properties across their surface. Application of photometric models to yield the physical properties of airless planetary surfaces requires observations spanning the broadest possible range of viewing and illumination geometries, from full disk to thin crescent. While New Horizons observed the Pluto system at phase angles no smaller than 15°, full-disk Hubble Space Telescope images (HST Program 13367, M. Buie, PI) at phase angles between 0.06° and 1.7° constrain the phase function near opposition. Together with New Horizons disk-resolved images, we use the near-opposition phase function to test whether ejecta exchange (Stern 2009, Icarus 199, 571) affects photometric properties. With direct measurements of small satellite sizes from New Horizons, we can determine the disk-integrated geometric albedo by extrapolating the HST observations to zero phase. New Horizons observations at larger phase angles reveal surface roughness and directional scattering properties. Finally we place the photometric properties of Pluto’s small satellites in context with those of similar size in other planetary systems. This work was supported in part by the NASA New Horizons Project.