Enceladus August 11, 2008 encounter, Close-up observations of plume vents Options

[ugordan]

As I understood it, Cassini's rotation won't be able to cancel out Enceladus' motion at all. They'll spin the spacecraft as fast as possible and wait for the moon to overtake the camera FOVs. This will at least lower the motion smear somewhat.

I'm very, very skeptical any plume will be visible at roughly a 90deg phase angle, looking down on sunlit (albeit terminator) area. I don't believe that's even the goal.

You can pretty much forget about any "movies" from such a close range. The C/A part of the encounter is practically gone within a minute and the fastest the ISS camera can take frames is IIRC every 20 seconds, and that's using at least 2x2 binning, possibly lossy compression as well and probably the fastest telemetry pickup rate available to the instruments (in other words, it's the only instrument producing significant quantities of data - this probably also excludes "BOTSIM" ISS mode).

Shutter times for the WAC can be a few ms only, I think NAC needs at least 20-ish ms (thin air figure) for good S/N ratio at 90deg phase icy terrain illumination.

[jmknapp]

As I understood it, Cassini's rotation won't be able to cancel out Enceladus' motion at all. They'll spin the spacecraft as fast as possible and wait for the moon to overtake the camera FOVs. This will at least lower the motion smear somewhat.

I was going by this quote:

"A special spacecraft maneuver implemented for this flyby first points Cassini's ORS platform ahead of Enceladus while the spacecraft is spun at its fastest speed in the direction that Enceladus moves across the sky. When Enceladus' apparent motion eventually overtakes the spacecraft spin and passes in front of the ISS cameras, the relative motion of the camera boresight across the surface of Enceladus will be briefly matched well enough so that the ISS NAC will be able to obtain seven very high resolution snapshots of selected geological features."

So call it "briefly matched well enough."

Thanks for the info on camera specs.

Here's yet another animation, concentrating on the ISS pointing for the half-hour or so after closest approach, ending just before the eclipse:

Enceladus Aug. 11 21:06-21:36 (67MB mpg file)

That one runs at about 10x real-time (compared to 90x for the first animation).

[volcanopele]

The skeet shooting images will be full-frame images, actually. Exposure times are on the order of 15-35 ms, though I have seen lower for the NAC.

[Holder of the Tw...]

I'm very, very skeptical any plume will be visible at roughly a 90deg phase angle, looking down on sunlit (albeit terminator) area.

Speculation here: might we see a plume shadow, similar to the martian dust devil shadows we see in pics from mars orbiters?

[ugordan]

I don't believe a shadow can/will be visible, the plumes are much more subtle than martian dust devils are and the low exposures will likely add a bit of noise to the images as well. The fact the plumes are only visible at high phase angles means they don't intercept much light in the first place. What is intercepted is mainly scattered, otherwise the plumes would appear dark.

Also, keep in mind the solar illumination will be coming in at a low angle so topography will be very pronounced, it would be much harder to spot shadows on that than a smooth martian surface.

[jmknapp]

How about plume material that might be up in the sunlight, but silhouetted against the night side?

[ugordan]

That would be potentially visible in an ideal situation, but again, there are two problems: the phase angle is still too low and scattered light off the sunlit surface might swamp out any signal from the plumes. I don't think I've ever seen a clear sign of the plumes at a 90 deg phase angle in distant Enceladus imagery.
It'd be nice to be wrong on this, though!

[elakdawalla]

Here's some more good stuff on the WAC part of the closest-approach imaging, from Anne Verbiscer, who wrote the looking ahead feature on the ciclops site:

The WAC terminator image has a few purposes: it's still a pretty high resolution... 48 meters/pixel (in the WAC!); it will catch some portion of the sunlit side of the terminator.... and will contain the first sunlit pixels after closest approach. Truth be told, there is actually a BOTSIM *before* this WAC, totally in the dark... who knows what will show up in that.... I didn't talk about it in the writeup because it's too difficult to explain. Basically, it's done to set up the other footprints' timing. It's literally a "shot in the dark"...

Neato

--Emily

[ugordan]

So there's a wide-angle shot over the limb while no sunlit terrain is visible? If any image shows the plumes, it will be that one! I can't wait!

[Anne Verbiscer]

So there's a wide-angle shot over the limb while no sunlit terrain is visible? If any image shows the plumes, it will be that one! I can't wait!

Sorry, ugordan, it's a WAC over the terminator, not limb. After ring-plane-crossing in 2009, the plumes will still be sunlit while the surface will be in darkness, providing just the opportunity you describe for ideal plume imaging.

[jasedm]

Good to see the interest that this flyby has garnered - guess we'll all have to wait and see
My vote for close-up plume shots/movies would be the later (equatorial flybys) - back-scattered light on the outbound leg perhaps.....

[Stu]

For a change this is pretty good timing for us Brits... Anyone wanting to stay up late to follow the fly-by will be able (weather permitting) to catch one of the year's best meteor showers too.

The Perseid meteor shower is due to peak at 06.00BST on the morning of August 12th, so anyone staying up late the previous evening will - again, weather permitting - see more shooting stars than usual, and by the time the sky brightens early a.m. next morning we might be getting some Enceladus images, maybe? So, my timetable looks like it's set... finish work at 10, online by 10.20 to follow the fly-by here, go outside around 11.30 to watch the meteor shower, back inside at around 04.00 and check how the fly-by went, head off to bed 6ish... Hmmm, I like the sound of that plan.

[volcanopele]

I got 0530 UTC for images to show up on the JPL Raw images page.

[Stu]

I got 0530 UTC for images to show up on the JPL Raw images page.

Thanks VP, appreciate that tip. Looks like bed at 7 instead of 6 then

[Anne Verbiscer]

For those interested primarily in the close approach imaging (including the Skeet Shoot), that playback won't start until nearly 1800 UTC on 12 August.