Simple theoretical calculations have suggested that small body impacts onto Pluto ’ s 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 ( 14 ; 10 ) ; 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 .