New Horizons Investigations of Charon and Pluto's Small Moons

During the flyby of the Pluto system in July 2014, the instruments on the New Horizons spacecraft (Weaver et al. 2008, Space Sci. Rev. 140, 75) acquired spatially resolved measurements of Charon and Pluto's small moons (Styx, Nix, Kerberos, and Hydra). The sunlit hemisphere of Charon was mapped in panchromatic light with resolutions as high as 0.15 km/pix using LORRI, and in four different color bands (400-550 nm, 540-700 nm, 780-975 nm, 860-910 nm; the latter is centered on a weak CH4 band) with resolutions as high as 1.4 km/pix using MVIC. Composition maps of Charon were obtained with the LEISA infrared spectral imager in the wavelength range 1.25-2.50 microns, with a spectral resolving power of ~250 and with spatial resolutions up to 4.9 km/pix. Solar occultation observations with the Alice ultraviolet spectrograph, and radio occultation measurements with REX, were used to search for an atmosphere around Charon. Nix was observed by LORRI in panchromatic light at 0.30 km/pix, by MVIC in color at 2.0 km/pix, and by LEISA at 3.6 km/pix (the latter to be downlinked later). Hydra was observed by LORRI in panchromatic light at 1.1 km/pix, in color at 4.6 km/pix, and by LEISA at 14.9 km/pix (the latter to be downlinked later). Limited resolved measurements of Kerberos (2.0 km/pix panchromatic; 8.0 km/pix color) and Styx (3.2 km/pix panchromatic; 8.0 km/pix color) were also obtained but have not yet been downlinked. An extensive series of unresolved, photometric measurements of Pluto's small moons were obtained with LORRI during several months preceeding closest approach in mid-July, which place tight constraints on their shapes and rotational states.The New Horizons data have revealed that Charon has surprisingly diverse terrain, with evidence of tectonics and a heterogeneous crustal composition. Nix and Hydra are highly elongated bodies with high average albedos (suggesting water-ice dominated surfaces) and significant albedo and color variations over their surfaces. We will present estimates for the densities, spin periods, and physical properties of all the small satellites. This work was supported by NASA's New Horizons project.