2007-09-10 Iapetus (rev 49): Pre-flyby discussion, Closest approach of this odd moon Options

[scalbers]

I'll try attaching a quick Celestia movie if it can fit in the size limit...  iapetus6.avi ( 481.5K ) Number of downloads: 967

[scalbers]

Here's a second version of the movie that focuses more on the closest approach...

 iapetus7.avi ( 475K ) Number of downloads: 959


EDIT:
I've made a longer version of the movie (3MB) as well at the link shown below. Celestia has the closest approach too far away at about 5000km, though at farther ranges the animation should be fairly realistic.

http://laps.noaa.gov/albers/sos/saturn/iapetus/iapetus9.avi

[MarcF]

Some more informations about the September flyby :

http://www.cosis.net/abstracts/EPSC2007/00...076ebb8f800aea1

A SAR observation will be performed, the only one of an icy satellite.

Will it help to determine the thickness of the dark material (or reveal hidden structures) ?

Marc.

[tasp]

Wow !!

Perhaps we can characterize the surface roughness for a possible Iapetan rover someday with a SAR observation.

I would love to see a scan of Atlas too, maybe it would reveal new info on the accumulated ring materials.

[scalbers]

A fast moving view covering the entire month before encounter, then some time afterwards...

Attached File(s)

 iapetus12.avi ( 666K ) Number of downloads: 258

 

[Decepticon]

Are there night side images planned?

[nprev]

I'm thinking that the radar obs are going to be primarily conducted during C/A for surface mapping, mostly due to the fact that it will be dark there...good use of resources!

[TritonAntares]

Are there night side images planned?

As far as I'm informed there'll be some attempts to catch the Snowman at the western limb hours after C/A.

5 images along the terminator (blue line) and 2 pics of the 3 craters in saturnshine area.

Before C/A there is no possibility of saturnshine images, because CASSINI is observing the anti-saturn hemisphere, I guess.
A perfect opportunity for radar mapping...


Bye.

[Rob Pinnegar]

There probably wouldn't be much point in trying to get Saturnshine images of the Snowman. The last set required some pretty long exposures, but since they were taken from 100 000 kilometres out, that didn't cause any problems.

For this encounter, though, Cassini will be so much closer in that any attempt to take Saturnshine images would probably lead to some awful smudging. This would be due to changes in the apparent size and shape of image features during the course of the exposure -- so tracking the camera wouldn't help.

So using the SAR instead seems like a really good idea.

[TritonAntares]

There probably wouldn't be much point in trying to get Saturnshine images of the Snowman. The last set required some pretty long exposures, but since they were taken from 100 000 kilometres out, that didn't cause any problems.
For this encounter, though, Cassini will be so much closer in that any attempt to take Saturnshine images would probably lead to some awful smudging.
This would be due to changes in the apparent size and shape of image features during the course of the exposure -- so tracking the camera wouldn't help.

They'll definitly try to do these two saturnshine shots with a resolution of ~140 m/pxl (NAC).
Maybe somebody can calculate the appendent distance.
We'll see how smeared they actually are. As this is the only opportunity to have a close look at the Snowman, it should better not be missed.
A bad pic is better than none...
Btw., the eastern edge of Snowman C will be in sunlight at the limb.

The dark green lined 15-image-mosaic will be taken hours later at ~440 m/pxl (NAC).

Bye.

[Rob Pinnegar]

I suppose it'll be okay as long as the images are taken long enough before closest approach. I guess that when Cassini is near closest approach, there won't be time for long exposures, anyways.

It'll be interesting over the next 40 days to see Iapetus get closer and closer.

[scalbers]

Possibly even daylight images would be challenging right at closest approach. Is it true that the closest imagery planned is for the Voyager mountains? If so what would be the range of Cassini for those images?

[ugordan]

140 m/pix translates into roughly 23 000 km range. Alternatively, if they plan on doing a 2x2 binning mode (useful if smear is expected to be >1 pix) that would be halved. A 2x2 bin has an additional advantage in that it increases the s/n ratio 4 times for the same exposure, but at the expense of spatial resolution. Even at 23 000 km the smear would probably less than one pixel for an exposure on the order of a couple of seconds.

Steve, I don't think daylight images will present a problem for Cassini. Remember the flyby speed will be much lower than typically at Titan (6 km/s) and especially Enceladus. Cassini managed to keep pretty stable pointing & tracking for the haze-penetrating CB3 filter even at those speeds and even with thrusters. The CB3 filter requires quite a long exposure to achieve good s/n ratio so this is indicative of the performance we can expect at a more leisurely flyby.

[scalbers]

Hi Gordan. Good points - I was thinking that the closest approach is around 1200km and that it'd be unlikely images would be right at that distance. There is some background info in post #7 about some close images that are so close the Voyager mountain targeting would be uncertain. Perhaps those are only at 2000km range that would be quite a bit closer than Enceladus.

[ugordan]

Ahh, I see post #7 has all the relevant information about saturnshine images as well. My impression from that post is that it's the targetting that's uncertain, not the camera's ability to produce smear-free images. Once targetted, Cassini can inertially track very well. In fact I'd say the limiting factor in the encounter will be the slow camera image rate and the lack of a scan platform for quick mosaicking several overlapping footprints a-la Galileo. Even if the NAC misses the white peaks, we'll still get pretty good context from the WAC I imagine. Remember the Rhea flyby when that 'splat' crater was of interest - even the WAC frame got some seriously high resolution at some 500 km distance.
A WAC frame at 1500 km is still 10x higher resolution than the best NAC coverage we got on New Year's Eve!