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

[Rob Pinnegar]

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.

Ohhhh, right -- I had forgotten about this when typing my earlier post.

The flyby geometry is really interesting. In early September, the apoapsis of Cassini's orbit will be just a little bit further out than Iapetus' distance from Saturn, but the semimajor axis of Cassini's orbit will of course be much smaller. This means that, when Cassini is furthest from Saturn, it will be a bit farther away from the planet than Iapetus, and will also be moving much more slowly than Iapetus.

This allows Cassini to pass "in front" of Iapetus on the outbound leg, and then watch as Iapetus overtakes it. Then Cassini drops back towards Saturn as Iapetus moves away. This allows quite a variety of observing angles during the whole encounter.

[scalbers]

Interesting - this might be worth a Celestia animation or something. Another aspect of the great planning is that while it is of course passing over the daylit side, Cassini is traveling around Iapetus in the same direction that each terminator is moving so we end up seeing more than 180 degrees of longitude that is sunlit over the encounter.

[Rob Pinnegar]

A little bit, yes. In the first few days after C/A, Cassini will recede from Iapetus at almost exactly 200 000 kilometres per day. You can check this easily in the Solar System Simulator.

[Edit, Aug 10th: The point of this is that Cassini will be fairly close to Iapetus for several days, so, as Steve noted above, we'll get a bit more than 180 degrees of coverage. I didn't make that very clear the first time.]

This means that we'll be getting about 14-15 hours' worth of images that will all be better than the ones from the New Year's flyby.

[mchan]

That would be great. Now only if there is sufficient data storage for Cassini to continue taking data instead of having to transmit earlier stored data to free up space.

[TritonAntares]

CICLOPS:
Cassini’s journey at Saturn continues with Rev 49, its 50th orbit of the ringed planet. Cassini has a full plate during this more than one-month-long orbit, as the spacecraft observes Saturn, its ring system, and four of its satellites: Titan, Tethys, Rhea, and Iapetus.
.....
Fast approaching is Iapetus. Cassini will perform its only targeted encounter with the two-toned moon on September 10. Several observations are planned for Iapetus as Cassini approaches, during the first 10 days of September.
Check back on September 6 for more details on this exciting encounter.

Onward to Iapetus!

Be patient, we'll have to wait a while...

[Rob Pinnegar]

This bit about Cassini making observations of Iapetus in the days leading up to the encounter must have to do with Saturnshine observations. So maybe we'll get a better look at Snowman after all.

[scalbers]

...and perhaps the Snowman and environs to the SE will get some interesting viewing angles after closest approach. Even though Cassini is moving away there are several days where the terminator is marching so we can watch the sunrise at Snowman.

And here's an initial take on a Celestia animation over the next month looking from above. The sunlight comes from the top with Cassini and Iapetus at the "C" and "I", respectively.
 iapetus16.avi ( 717K ) Number of downloads: 289

[ugordan]

Emily's latest blog entry has some info on the upcoming flyby, she apparently talked about it in detail with Tilmann Denk himself.

[MarcF]

Ices, oceans, and Fire: Satellites of the Outer Solar System (2007)

Tilmann Denk's abstract about the Iapetus flyby:

http://www.lpi.usra.edu/meetings/icysat2007/pdf/6049.pdf

Eleven ISS mosaics are planned for the time period -55 to +180 min around closest approach.
Spatial resolution down to 10 m/pxl with ISS.
And a lot of scientific highlights with all the other instruments (SAR, UVIS, CIRS VIMS, ...).
Less than one month to wait !!
Marc.

[scalbers]

I was also at the Outer Planet Satellite conference - a local one for me in Boulder. I had a chance to talk about Iapetus a bit with Tilmann Denk (and saw Emily there as well). One item relevant to targeting is how close the Voyager mountains are to the equator. The yellow outlined footprints (post #32) are centered just a bit south of the equator. In my map (that could be off a couple of degrees), they are south of the equator in the southern part of the imaging footprint. Tilmann thinks they may actually lie more exactly on the equator that would bring them up into the northern part of the footprint. Either way, hopefully they will indeed show up in that rather high resolution mosaic.

He mentioned they have some ability to tweak the timing of the imaging sequence until very near the encounter based on the latest navigation updates. There was a very nice table of the images on his poster that Emily alludes to in her blog.

Tilmann had on his poster a few RGB filter images. One of these was from the July 2007 encounter showing the Roncevaux Terra basin region near the limb having nice contrast without the JPEG artifacts. We also had a good exchange about the "basin gap" question, though perhaps we'll have to wait until the September imaging to reach a consensus about what is really going on in terms of possible double rims/basins

[EDITED Aug 18 1600 UTC]

[Tom Tamlyn]

Tilmann Denk's abstract about the Iapetus flyby:
http://www.lpi.usra.edu/meetings/icysat2007/pdf/6049.pdf

Thanks for posting that url. Quoting Denk's abstract:

>the only SAR observation of an icy satellite (RADAR); *** best-ever examination of
>outer-solar system dark material (VIMS);

I haven't been following the icy satellites as closely as many UMSF members, but it seems odd that this is the only time that SAR is being used to observe an icy satellite. If that's correct, what's the reason?

And what is "outer-solar system dark material?" Is that just another way of referring to the dark side of Iapetus?

TTT

P.S. I'll take this opportunity to thank Emily for her indefatigable blogging of the Ices, Oceans & Fire conference in Boulder. Much appreciated. Why can't they schedule one of these conferences for NYC?

[remcook]

I think the dark material is also for instance the stuff found on Phoebe:
http://www.nasa.gov/lb/mission_pages/cassi...a/pia06400.html
but I'm not sure...

"Why can't they schedule one of these conferences for NYC"
next year's DPS is in Ithaca, which is as close as you're going to get it I guess.

[TritonAntares]

Hi,
the following footprint-map (taken from Tilmann Denk's above mentioned abstract) shows a 4x3 and 3x3 NAC-mosaic of 82-131 m/pxl resolution:


Any idea of the WAC-resolution of the large field on this map?
This image could be crucial for major findings in this transition region, as the area is over 3 times larger than the one covered by the mosaic.

THX & Bye.

[TritonAntares]

Hello again,
there will be 4 footprints of the Iapetus cresent (map also from Tilmann Denk's abstract) taken hours before C/A at about 490 m/pxl:


Here what Solar System Simulator shows for the time window of the 4 images:

Distance will be between 70000 and 69000 km.

I'm just wondering about the saturnshine hemisphere...

If I'm right, Saturn should illuminate the 'backside' of the of Iapetus at inferior conjunction,
thus the hemisphere shown at superior conjunction (270°-90°) - most of Cassini Regio and the Snowman.

There seems to be no try to catch parts of this saturnlit hemisphere before C/A -
I assume the cresent images won't be overexposed - from this distance !?
Otherwise, there do exist numerous pics of Cassini Regio by now.

Bye.

[Rob Pinnegar]

Regarding the "basin gap" question, check out this old abstract by Denk et al:

http://ciclops.org/media/ma/2007/2748_7462_0.pdf

If you look on Page 2, the second figure from the top shows several low-resolution, distant shots of the Roncevaux Terra basin during a multi-day sunrise.

I can't see a double-basin rim, myself.