New Horizons Ring Collision Hazard: Constraints from Earth-based Observations

The New Horizons spacecraft's nominal trajectory crosses the planet's satellite plane at $\sim 10,000\ \rm{km}$ from the barycenter, between the orbits of Pluto and Charon. I have investigated the risk to the spacecraft based on observational limits of rings and dust within this region, assuming various particle size distributions. The best limits are placed by 2011 and 2012 HST observations, which significantly improve on the limits from stellar occultations, although they do not go as close to the planet. From the HST data and assuming a `reasonable worst case' for the size distribution, we place a limit of $N < 20$ damaging impacts by grains of radius $> 0.2\ \textrm{mm}$ onto the spacecraft during the encounter. The number of hits is $\approx$ 200$\times$ above the NH mission requirement, and $\approx$ $2000\times$ above the mission's desired level. Stellar occultations remain valuable because they are able to measure $N$ closer to the Pluto surface than direct imaging, although with a sensitivity limit several orders of magnitude higher than that from HST imaging. Neither HST nor occultations are sensitive enough to place limits on $N$ at or below the mission requirements.