This Wednesday, October 28 2015 at 11:22 a.m. EDT (15:22 UTC), Cassini spacecraft will flyby through the Enceladus plume at only 30 miles (49 km) altitude to analyze plume chemistry with its ion and neutral mass spectrometer (INMS). While the plume particle size is estimated at only 30 microns, and it is fairly tenuous, relative velocity on flyby E-21 will be 19,014 mph (8.5 km/sec)!
Animation of Cassini's Oct. 28, 2015 flyby of Enceladus, during which the spacecraft will make its deepest dive through the moon's active plume of icy material.
My question is, how risky has this been assessed to be to the spacecraft and its instruments?
It seems to me that even if the momentum of the plume particulate during the flyby, due to their low mass and density, isn't sufficient to cause, say, sizable dents and tearing of the spacecraft's multi-layer insulation, since Cassini will also attempt to take close proximity images of Enceladus' South polar region where these vents are forming the plume, this could cause darkening of optics to occur through contact with ionized molecular hydrogen and other trace gasses in the plume. Especially since Enceladus will only be illuminated by Saturn's shine, which should require longer exposure times. And perhaps it could cause other problems that I haven't envisioned here.
I'm sure JPL did risk assessment, so I'm mostly interested in any published papers discussing this flyby in greater detail from perspective of risk management.