New Satellite of (134340) Pluto: S/2012 (134340) 1

M. R. Showalter, SETI Institute; H. A. Weaver, Applied Physics Laboratory, Johns Hopkins University; S. A. Stern, A. J. Steffl, M. W. Buie, and W. J. Merline, Southwest Research Institute; M. J. Mutchler and R. Soummer, Space Telescope Science Institute; and H. B. Throop, NASA Headquarters, report the discovery of a fifth satellite of Pluto. The object, provisionally designated S/2012 (134340) 1, was detected in fourteen separate sets of images taken by the Hubble Space Telescope WFC3/UVIS. Each image set comprises eleven to twelve 3-minute exposures. Upon co-adding, S/N = 5-8 in five sets and S/N = 3-5 in nine sets where the detection was somewhat degraded by the satellite's close proximity of Pluto II (Nix). Times and offsets for the new satellite are as follows: June 26.51-26.67 UT, 3 sets, 1".99 from Pluto in p.a. 158 deg; June 27.78-27.94, 3 sets, 1".71 in p.a. 182 deg; June 29.64-29.80, 3 sets, 1".44 in p.a. 219 deg; July 7.42-7.58, 3 sets, 1".76 in p.a. 352 deg; July 9.41-9.51, 2 sets, 1".42 in p.a. 31 deg. The satellite's mean magnitude is V = 27.0 +/- 0.3, making it 4 percent as bright as Pluto II (Nix) and half as bright as S/2011 (134340) 1. The diameter depends on the assumed geometric albedo (10 km if p_v = 0.35, or 25 km if p_v = 0.04). The motion is consistent with a body traveling on a near-circular orbit that is co-planar with the other satellites. The inferred mean motion is 17.8 +/- 0.1 degrees per day (P = 20.2 +/- 0.1 days), and the projected radial distance from Pluto is 42000 +/- 2000 km, placing S/2012 (134340) 1 interior to Pluto II (Nix) and close to the 1:3 mean motion resonance with Pluto I (Charon).