R.L. wrote:
Does anyone know what the contamination is? I personally assume it must be outgassing from some spacecraft component...

Alex writes:
This is plausible, but as you noted almost all spacecraft (including Cassini) undergo extensive preflight testing in vacuum chambers. Although Bob Mitchell, Cassini's Project Manager, seems to discount thruster plume contamination in this case, one will note that all of the recent optics contamintation experienced by interplanetary spacecraft (Cassini, NEAR Shoemaker, and Stardust) have happened on spacecraft with no moveable scan platforms (i.e., instruments cannot be pointed and/or stowed from the effects of thruster plumes as safely as they might have been before). NEAR Shoemaker's problems came from thruster contamination associated with the aborted rendezvous burn in 1998, though image processing solutions were able to ameliorate (and almost totally eliminate) the problem. Stardust's contamination is thought to have originated from contamination during the spacecraft's separation from the kick stage, where volatiles are suspected to have migrated across the thermal gradient and were cold trapped on the shaded optics. Despite the Stardust Project's assertion that the problem has been "totally solved", one will note that no engineering images have been released since the (rather lousy) lunar image taken shortly after the Earth gravity assist flyby earlier this year:

Note also that this image was taken *after* the Stardust Project claimed to have "...improved the spacecraft's navigation-camera resolution to nearly normal." I can only speak for myself and a few colleagues, but this lunar image hardly looks "normal."

At any rate, modeling of contamination from thrusters is not as "exact" as one might believe. Based on the above, I would have to say that this scenario for Cassini is just as plausible as not.

Alex R. Blackwell
University of Hawaii