Simple theoretical calculations have suggested that small body impacts onto Plutos newly discovered small satellites, Nix and Hydra, are capable of generating time-variable rings or dust sheets in the Pluto system. Using HST/ACS data obtained on 2006 February 15 and 2006 March 2, we find no observational evidence for such a ring system and present the first constraints on the present-day I/F and optical depth of a putative ring system. At the 1500-km radial resolution of our search, we place a 3-sigma upper limit on the azimuthally-averaged normal I/F of ring particles of 5.1x10^-7 at a distance of 42,000 km from the Pluto-Charon barycenter, the minimum distance for a dynamically stable ring (Stern et al., 1994; Nagy et al., 2006); 4.4x10^-7 at the orbit of Nix; and 2.5x10^-7 at the orbit of Hydra. For an assumed ring particle albedo of 0.04 (0.38), these I/F limits translate into 3-sigma upper limits on the normal optical depth of macroscopic ring particles of 1.3x10^-5 (1.4x10^-6), 1.1x10^-5 (1.2x10^-6), 6.4x10^-6 (6.7x10^-7), respectively. Were the New Horizons spacecraft to fly through a ring system with optical depth of 1.3x10^-5, it would collide with a significant number of potentially damaging ring particles. We therefore recommend that unless tighter constraints can be obtained, New Horizons cross the putative ring plane within 42,000 km of the Pluto-Charon barycenter, where rings are dynamically unstable. We derive a crude estimate of the lifetime of putative ring paritcles of 900 years.
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