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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.

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  • $\begingroup$ If I've done my calculations correctly, impact of a single 30µm water droplet at 8.5km/s relative speed delivers ~0.5 milliJoules of kinetic energy -- about the same as a 2mm water drop falling from a one meter height on the surface of the Earth, so individually they shouldn't be much of a concern. $\endgroup$
    – Russell Borogove
    Oct 26, 2015 at 18:48
  • $\begingroup$ @RussellBorogove Read on, my concern is also darkening of optics. That is a real effect and something say SOHO/SDO have to regularly calibrate for (and that's darkening by much more tenuous tho higher velocity solar wind. If you check Cassini raw images, you'll notice quite a lot of deformation and specs on its filters, for example. It's especially apparent with long exposures. 2 mm water drop falling from one meter height might not damage your telescope, but you'll still want to clean that off before you use it next time. ;) $\endgroup$
    – TildalWave
    Oct 26, 2015 at 18:56
  • $\begingroup$ Tiny windshield wipers. $\endgroup$
    – Russell Borogove
    Oct 26, 2015 at 19:13
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    $\begingroup$ I can imagine the engineers accepting higher risk levels because we are now in the "Cassini Grand Finale," which will end in September 2017 with its plunge into Saturn's atmosphere. $\endgroup$
    – user10509
    Oct 28, 2015 at 15:36

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I don't have the source for this latest flyby, but I do have one for the 50 km pass that was previously executed in 2008, only slightly further than the 49 km pass this go-around. The two stated dangers were "The two threats to the spacecraft were identified to be an inadvertent impact with Enceladus, and damage resulting from the environment within the plume." The impact with Enceladus was deemed to be very unlikely. As for the particles, they used data to determine there wasn't any particles of sufficient size in the plume to damage the spacecraft. Optics were not included in the publicly released danger considerations.

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  • $\begingroup$ Yeah thanks for this but I'm really interested in that tradeoff study that your link mentions indirectly in: Therefore 50 kilometers was chosen as an appropriate tradeoff between optimal science and risk to the spacecraft.. This source also doesn't discuss plume density so I can't assess risk relative to 2008 flyby. Judging by latest photos I can find, it seems to me that Enceladus is a bit more active this time around than in 2008 when the plume wasn't even noticeable on images (tho lighting wasn't favorable). $\endgroup$
    – TildalWave
    Oct 27, 2015 at 18:32
  • $\begingroup$ I'm not sure how long stored query will last, but here's a link to February - April 2008 snaps of Enceladus. In other words, I'm not sure how to compare risk of these two flybys. $\endgroup$
    – TildalWave
    Oct 27, 2015 at 18:34
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I wish NASA would make transcripts of their audio broadcasts, because this question was raised by someone in the media during the question & answer part of last Monday's teleconference. (I searched, couldn't find any transcripts -- can someone do better?) In short, and working from memory, there's apparently no risk at all to the spacecraft or its instruments. I remember the flight director saying something along the lines of, "we know where Enceladus is and where Cassini is" and the risk to the spacecraft is virtually nil.

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    $\begingroup$ I posted links to Monday's teleconference at the end of this answer, if that helps. Tho note that I tagged my question as reference-request and am primarily interested in any tradeoff study based on which this assessment presented during the presser was made. Of course, just the rundown helps, but I'd really like to look deeper into it than what's been discussed during the presser (basically, those are predigested information). $\endgroup$
    – TildalWave
    Oct 28, 2015 at 15:30

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