T38 (Dec 05, 2007) Options

[Del Palmer]

The flyby mission description is now up.

1,300 km altitude, southern hemisphere, outbound. ISS and VIMS will observe Ontario Lacus.

[ngunn]

The new 'Looking Ahead' has also appeared on Ciclops. Edit - info already posted by VP, sorry.

Unusually close-up ISS view of Ontario Lacus may have some smear. Looking forward to it all the same!

[belleraphon1]

All...

been eagerly awaiting these south polar passes. I believe most of the cloud activity that was so prevalent in 2004 has abated at the pole. Wonder what VIMS and ISS will see there now.

Also be neat to see what VIMS and ISS make out for Ontario Lacus... can VIMS pin done a liquid phase for the dark material? From the sample snapshots looks like CIRS will try for a temperature reading.

And I think T39 (December 20th) has RADAR SAR over the southern hemisphere.

Great Holiday treats on the way!!!!!

Craig

[JRehling]

Also be neat to see what VIMS and ISS make out for Ontario Lacus... can VIMS pin done a liquid phase for the dark material? From the sample snapshots looks like CIRS will try for a temperature reading.

Any changes at all would be fascinating. If there's liquid in any of the darkish areas, and the seasons have changed the aquifer level, we should see it with ISS/VIMS.

Ontario Lacus may be fairly resistant to such changes -- those crisp boundaries might mean some pretty well-defined banks -- but other darkish areas had very amorphous boundaries, and a slight change in aquifer volume could translate to big changes in surface area submerged.

Overall, it seems like there are a few kinds of topography at the poles translating into very different shorelines. Part of the northern polar area looks like Minnesota (many lakes), part looks like Kazakhstan (a few huge seas). The south seems to have just Ontario Lacus as an example of crisp, curvy shorelines, with other features perhaps featuring very shallow lakes that transition gradually into swamps.

[volcanopele]

The T38 flyby is now the top story on the Cassini website. You might notice a disconnect between the description the header images uses, "Cassini's Search for Subsurface Ocean Continues," and the goals of the flyby: to image Ontario Lacus. Originally, the T38 encounter was to be a "gravity pass", with the Radio Science team having time at closest approach and at other times in the encounter to measure the gravitational field of Titan. Combined with three other passes in the nominal mission, the interior structure and the thickness of Titan's internal ocean could be discerned. After the discovery of Ontario Lacus, it became important to determine whether it was in fact a lake filled with liquid methane. Cassini's trajectory during T38 takes it directly over Ontario Lacus. The VIMS team successfully argued for a change in the T38 sequence to make it an ORS pass, giving prime targeting control to VIMS during C/A. However, many internal documents have not been updated to reflect that change (including my copy of the timeline, as you can tell from the CICLOPS looking ahead page, which still mentions the non-C/A RSS passes).

Which might explain why the graphic used on the Cassini homepage says that this pass will continue the search for an internal ocean.

[ngunn]

Could somebody dispel a little bit of my ignorance? Why is it that VIMS can do imaging at closest approach (less that 2000 km.) while ISS images taken closer than 40,000 km are 'often smeared'?

[paxdan]

VIMS has a much lower spacial resolution per pixel.

[ngunn]

Thanks paxdan. Presumably also there is no way to 'stop down' the ISS camera so that it can take smaller detailed shots at closer ranges of things like the Huygens landing site and lake shores?

[rlorenz]

VP - it wasnt just the VIMS team.
One of your ISS colleagues close to me was also among the chorus (including myself) that
argued for T38 being used for ISS and VIMS to observe Ontario, it being a particularly good
opportunity to do so. (Radar will hit it in XM, hopefully with both SAR and altimetry - the illumination
will be poorer then, but we dont need no stinkin sunshine...).

A major factor in this re-assignment was the calculation (once all the
details of ground station availability etc. were in and the error correlation matrices calculated) was that
the incremental value of the flyby towards the gravity field (and tidal variation thereof) determination
was in fact rather small. RSS will get better results from T45 in XM (assuming XM gets approved)

The T38 flyby is now the top story on the Cassini website. You might notice a disconnect between the description the header images uses, "Cassini's Search for Subsurface Ocean Continues," and the goals of the flyby: to image Ontario Lacus. Originally, the T38 encounter was to be a "gravity pass", with the Radio Science team having time at closest approach and at other times in the encounter to measure the gravitational field of Titan. Combined with three other passes in the nominal mission, the interior structure and the thickness of Titan's internal ocean could be discerned. After the discovery of Ontario Lacus, it became important to determine whether it was in fact a lake filled with liquid methane. Cassini's trajectory during T38 takes it directly over Ontario Lacus. The VIMS team successfully argued for a change in the T38 sequence to make it an ORS pass, giving prime targeting control to VIMS during C/A. However, many internal documents have not been updated to reflect that change (including my copy of the timeline, as you can tell from the CICLOPS looking ahead page, which still mentions the non-C/A RSS passes).

Which might explain why the graphic used on the Cassini homepage says that this pass will continue the search for an internal ocean.

[ugordan]

Presumably also there is no way to 'stop down' the ISS camera so that it can take smaller detailed shots at closer ranges of things like the Huygens landing site and lake shores?

"Stop down" as in 2x2 binning? That's regularly been done when smear was expected to be great or S/N ratio too small. Pointing at C/A is a greater issue I'd think.

[ngunn]

"Stop down" as in 2x2 binning? That's regularly been done when smear was expected to be great or S/N ratio too small. Pointing at C/A is a greater issue I'd think.

Ahh, interesting. Does 2x2 binning actually quarter the time needed to record an image? I was thinking of just using a small part of the detector array at full resolution. I'm a complete ignoramus as regards the instrumentation so if I'm asking stupid questions please just ignore!

VP and rlorenz - Thank you both very much for sharing this illuminating inside story of the Cassini team's continual debate and review of plans in the light of new discoveries and circumstances. It will be wonderful to see those lake pictures, and well worth a little temporary confusion on the public website.

[ugordan]

Ahh, interesting. Does 2x2 binning actually quarter the time needed to record an image?

It effectively quarters the exposure time needed for the same S/N ratio because you sum up 4 pixels. It also enables you to shoot somewhat faster (not 4x times faster, though), depending on telemetry pickup rates since you have less data to send to the recorders. Not insignificant with Cassini's slow framerates.

[remcook]

raws seem to be appearing at this very minute

[ngunn]

Indeed. More than 500 within the last hour it seems! Many, many beautiful details here. I didn't spot Ontario Lacus on my first skim through though. Anybody else find it? BTW what happens if they post more than 500 in one go? Does that mean that some aren't viewable?

[ngunn]

On closer inspection I notice that this was taken from approximately the right distance:
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=137342