The small satellites of Pluto as observed by New Horizons.

摘要: INTRODUCTION The Pluto system is surprisingly complex, comprising six objects that orbit their common center of mass in approximately a single plane and nearly circular orbits. When the New Horizons mission was selected for flight by NASA 2001, only two largest were known: binary dwarf planets Charon. Two much smaller moons, Nix Hydra, discovered May 2005, just 8 months before launch spacecraft, even Kerberos Styx, 2011 2012, respectively. entire likely produced aftermath giant impact between Pluto-sized bodies 4 to 4.5 billion years ago, with small moons forming within resulting debris disk. But many details remain unconfirmed, results on Pluto’s help elucidate conditions under which formed evolved. RATIONALE are difficult observe from Earth-based facilities, most basic visible near-infrared photometric measurements possible date. flyby enabled whole new category moons. Long Range Reconnaissance Imager (LORRI) provided high–spatial resolution panchromatic imaging, thousands pixels across surfaces Hydra first resolved images Styx. In addition, LORRI used conduct systematic monitoring brightness all four over several months, detailed rotational properties could be deduced. Multispectral Visible Imaging Camera (MVIC) color Hydra. Linear Etalon Spectral Array (LEISA) captured spectra (in wavelength range 1.25 2.5 μm) compositional studies, but those data have not yet been sent Earth. RESULTS All highly elongated high surface reflectances (albedos) suggestive water-ice composition. appears double-lobed shape, possibly merger bodies. Crater counts imply ages at least years. mostly neutral (i.e., gray) colors, an apparent crater Nix’s redder than rest surface; this finding suggests either impacting body had different composition or material excavated below surface. periods shorter orbital periods, poles clustered orthogonal direction CONCLUSION exhibit rapid rotation large obliquities, indicating tidal despinning has played dominant role evolution. Collisional processes implicated determining shapes collisional evolution probably limited hundred million after system’s formation. bright suggest if Pluto-Charon during collision, precursor partially differentiated icy layers.