[X] My Assistant

[angel1801]

Apart from Titan, will Cassini get close to any icy moons when it gets into the inner part of Saturn on or around April 30? I've checked using the Solar System Simulator and I have not seen anything good to date.

[tasp]

The Voyager trajectories with their multiple moon encounters on each of their respective passes spoiled us.

Lining up Titan on almost every pass really crimps the chance encounters with the other moons and moonlets.

[Guest_Analyst_*]

Almost all of the Voyager encounters were far encounters (or untargeted in todays terms). I have to dig out some papers, but I believe the Voyager 1 Titan encounter has been by far the closest (5.000 km). At Jupiter it was Io on Voyager 1 (20.000 km), at Uranus Miranda with 28.000 km and Triton at Neptune with 40.000 km. All numbers are from my memory and may be off by a few thousand km.

Analyst

[ugordan]

From what I see, Cassini will fly pretty far from all inner moons, but there is a distant opportunity for imaging Rhea on April 28. From 450 000 km, Rhea would nicely fill the NAC field of view, as seen in this image. I'm not sure if any observations are planned, though. We might at least expect a few multispectral frames, judging by past experience.

EDIT: It turns out the Solar System Simulator takes the width of the 800x450 frame as FOV so Rhea will actually take up only half of the NAC field-of-view. For some reason I assumed the logic as in Celestia holds where the smaller frame dimension is taken when calculating the FOV...

Also, Analyst is right, Voyager flybys were much more distant than what Cassini considers a close approach. Voyager tour designers only had a single pass through the system so they had to get the most out of it, optimizing for fairly close encounters (you can't even call most of them "flybys" - they were that distant) with as many moons as possible. Cassini is here to stay so there isn't a pressure to cram as many flybys as possible per orbit. Which doesn't mean there weren't some nice orbits in the past -- see the Rev 21 and especially Rev 19 threads for examples of some very nice non-targeted imagery!

[edstrick]

Voyager 1's primary small moon target was Rhea. As the largest of the "small" sats, it was a logical candidate for closest inspection with muiti-frame mosaic coverage. We couldn't know in advance it was a whole lot of craters and not much else.

Voyager 2's primary small moon target was Tethys. We got on corner of a narrow angle frame on the limb during a closest encounter mosaic sequence that was mis-targeted because the spacecraft came out of a fields-and-particles observation roll maneuver 1 degree off in roll. *** NOT BECAUSE OF THE SCAN PLATFORM STICKING *** Ring plane crossing narrow angle images taken shortly after were mis-targeted for the same reason.

The scan platform failure occured shortly after ring plane crossing but was not directly connected with either event, just the progressive friction increase as lube worked out of the gears during sustained active high-slew-rate platform activity. (Like the friction increase in Spirit's wheel that was cured by running in reverse)

[angel1801]

Voyager 2's primary small moon target was Tethys. We got on corner of a narrow angle frame on the limb during a closest encounter mosaic sequence that was mis-targeted because the spacecraft came out of a fields-and-particles observation roll maneuver 1 degree off in roll.

Did Cassini image the encounter mosaic that Voyager 2 missed back on September 24, 2005?

[ugordan]

the spacecraft came out of a fields-and-particles observation roll maneuver 1 degree off in roll.

How did this happen? Isn't the star tracker supposed to figure out the precise attitude to a better precision? Basics of Space Flight says the Voyagers used a sun sensor in addition to the star tracker that followed a bright star (Canopus was suggested) located at right angles to the sun.
What exactly was the specification of attitude pointing accuracy for the mission? I seem to remember a value not much better than the NAC field of view?

[Guest_Analyst_*]

See page 35 (pdf-page 47) on The Voyager Neptune Travel Guide for pointing information.

edstrick, do you have some papers (links) about the Voyager encounter planning and actual performance? It's quite hard to find these things from the pre-internet time.

Analyst

[ugordan]

There's a rather elaborate description of the mission goals here that goes into details on specific things including the optimal flight paths for the two craft for the Jupiter/Saturn encounters. This is pre-flight estimates, though.

Neptune Travel Guide looks like a very interesting read (even if poorly scanned). It says the final scan platform pointing error is +/- 0.1 deg (2 sigma) which is +/- 1/4 of NAC FOV. All the more reason I'm puzzled by the 1 degree error.

[angel1801]

See page 35 (pdf-page 47) on The Voyager Neptune Travel Guide .

I gave the Voyager Neptune Travel Guide a brief look and I observed that the early look of Cassini featured a Voyager type scan platform. Some how later on, it got replaced by a bolted camera system.

[ljk4-1]

Neptune Travel Guide looks like a very interesting read (even if poorly scanned). It says the final scan platform pointing error is +/- 0.1 deg (2 sigma) which is +/- 1/4 of NAC FOV. All the more reason I'm puzzled by the 1 degree error.

If you can find a hardcopy, they have single frames on each page corner of
Voyager 2 encountering the Neptune system. If you rapidly flip each page
you can see a "movie" of this event. Similar to what Carl Sagan did in his
1985 book Comet with Halley's solar orbit.

[ugordan]

If you can find a hardcopy, they have single frames on each page corner of
Voyager 2 encountering the Neptune system. If you rapidly flip each page
you can see a "movie" of this event.

I noticed this in the PDF. Although I only came to T- 2 hours before C/A, it was pretty cool to see, using simple 3d visualization.

I gave the Voyager Neptune Travel Guide a brief look and I observed that the early look of Cassini featured a Voyager type scan platform. Some how later on, it got replaced by a bolted camera system.

Which, ironically, although it made competing observations difficult, actually made the spacecraft more precise and stable during target motion tracking.

[Guest_Analyst_*]

It says the final scan platform pointing error is +/- 0.1 deg (2 sigma) which is +/- 1/4 of NAC FOV. All the more reason I'm puzzled by the 1 degree error.

I guess some kind of gyro drift. A roll maneuver is always under gyro control with no star reference. The HGA is pointing to earth (sun sensor) and the star tracker loses it's guide star because it is locking perpendicular to the HGA (roll axis is going through the HGA). So you need the gyros to have iternal reference. Some miscalibration or error built up because of gyro drift and you end up with an error. But this is only a guess.

Analyst

[volcanopele]

The best non-targeted encounter ofthis orbit is that of Janus on Saturday UTC. we come within 209,000 km of this small satellite. Should provide the best imaging to date of this moon, with pixel scales better than 1.25 km/pixel, though near the end of the encounter, Janus will start to be occulted by the rings. Images from this encounter should be down early Sunday.

This encounter also provides a decent imaging opportunity of Rhea.

[angel1801]

I'm here to tell you that the first two images of Rhea are now available. The distance is about 481,000km and Rhea is centered about 32S, 75W in these images. This means the "Great White Splat" is visible in both images relased to date. THe Janus images are not in yet.

[TritonAntares]

Hi,
there could be the possibility to have a 'closer' look at Janus (diameter 181 km).
As I read, CASSINI should pass it in roughly 204.400 km on april 29th, the closest distance so far.

I tried to create a simulation with Solar System Viewer to check this info,
but unfortunately Janus isn't inclueded there...

Best Janus image so far:

Date: 2005/05/20
Distance: 357.000 km
Resolution: ~2 km/pxl
Let's wait if they actually took some pics...

Bye.

[dilo]

This image of Epimetheus+rings+Titan is really stunning!
http://cassinicam.com/isslookup/html/74264.html

[dilo]

Best Janus image so far:

Date: 2005/05/20
Distance: 357.000 km
Resolution: ~2 km/pxl
Let's wait if they actually took some pics...

Waiting for closer looks, this is the sharpened/contrast enhanced version:

Attached thumbnail(s)

 

[Guest_BruceMoomaw_*]

Interesting. Janus -- like the little Lagrange moons of Tethys and Dione, but unlike Epimetheus -- seems mostly devoid of craters. Is this just an illusion resulting from relatively low reaolution, or is there really a major difference, crater-wise, between Janus and Epimetheus? And if so, what could possibly have caused it?

[Phil Stooke]

It is an illusion, but not caused by low resolution. It's caused by that image being low phase (nearly full). So the craters are not shadowed enough to see clearly. Voyager images show lots of craters.

Phil

[edstrick]

The article on Voyager 2's Saturn encounter that had that information almost certainly said what caused the 1 degree (I think that's about what it was) roll error, but I do not recall what it was. I suspect it may have been gyro drift during the fields and particles roll maneuver, or as likely or more so a simple command or spacecraft-behavior prediction error.

Voyager's attitude control was distinctly sloppy compared with the narrow angle field of view, though nowhere near as wobbly as Mariner 10's. Mission planning shows neat grids of narrow angle mosaics across Mercury, but the actual image sequences show curves and and wanderings more than one field of view in a few cases.

Voyager's attitude control became considerably tighter due to very very hard work testing the limits of the hardware and tweeking the attitude control software after Saturn flyby in preparation for the low light level work at Uranus and even lower light levels at Neptune. The performance improvement they got was astonishing. Some image sequences at Saturn were image motion blurred, while much of the Triton imaging at Neptune was essentially rock steady.

[ugordan]

Janus UV3-GRN-IR1 composite, distance ~220 000 km:

The image on the right is enhanced to bring out color differences across the disk.

[ugordan]

Rhea from ~481 000 km, R+B composite with a synthetic green:

The image on the right was taken two months ago, also showing "the splat", but using UV3-GRN-IR1 filters. The distance at that time was around 350 000 km. Note the right-hand image has Rhea's north directly up, while the left-hand one was taken with the sunlight coming directly from left and is thus slightly rotated clockwise.

[alan]

Which moon is below the rings?
http://saturn1.jpl.nasa.gov/multimedia/ima...eiImageID=74345

[TritonAntares]

Which moon is below the rings?

Should be Janus' companion Epimetheus...

[volcanopele]

Should be Janus' companion Epimetheus...

Nope, the other moon in that pic is Prometheus.

There is also a great sequence of Pan images, best resolution images of that small moon as far as I know:

http://saturn.jpl.nasa.gov/multimedia/imag...0/N00060357.jpg

Looks like a walnut... Earlier, I've been hesitant about the hypothesis that the giant ridge on Iapetus is due to accreting ring material, based on Iapetus current distance from Saturn, that idea doesn't make much sense. But for Pan, yep, makes perfect sense.

[TritonAntares]

Nope, the other moon in that pic is Prometheus.

Prometheus (length 102 km at left) and Pandora (length 84 km at right):

-click to enlarge-
Date: 2005/10/29
Distance: Prometheus 483.500 km & Pandora 459.000 km
Resolution: 3 km/pxl

The figure of the unknown moon rather resembles Pandora...

-click to enlarge-

Bye.

[volcanopele]

Nope, Prometheus. prometheus is a bit more hot dog shaped than Pandora.

[TritonAntares]

Nope, Prometheus. prometheus is a bit more hot dog shaped than Pandora.

Ok.Prometheus....

[volcanopele]

Janus UV3-GRN-IR1 composite, distance ~220 000 km:

Great view. For reference, the bowl shaped crater just to the upper right of the center of the disk appears to be the same crater as the one near the bottom of the upper left image at http://www.ssc.uwo.ca/geography/spacemap/jaindex1.htm

[SigurRosFan]

It's Janus (above, 218,000 km distance) and Prometheus (379,000 km distance):

- PIA08192: So Close…

[jasedm]

Can anyone confirm whether Cassini is still assigned to get some closer views of Janus in the next couple of days? - It was originally planned to take some shots at around 141,000km on the 25th September I believe, which would make them the closest views yet of the moon. I have to say I'm very intrigued at what appears to be dark material at the bottom of some of the craters - similar to Hyperion.

[ugordan]

The recent Cassini significant events report states the following:

A Live Inertial Vector Propagator (IVP) update is planned for Saturn and Janus from DOY 268-272. At the Go/No-go meeting today it was confirmed that the update would only be required for the Janus vector.

So I'm assuming there will be Janus observations. I'm curious about these dark crater floors you speak of. Where do you see them, apart from topography shading?

Volcanopele also sort of confirms the observations here.

[jasedm]

Hi ugordan, the dark crater floors were intimated at in May 2005, following a single very low phase angle image of Janus taken from 357,000km:
http://photojournal.jpl.nasa.gov/jpeg/PIA07529.jpg
It was stated that there may be dark material being exposed following very energetic impacts - possibly a very different process is at work here than on Hyperion...

[ugordan]

Somehow I'm not very convinced those are dark crater floors in that image. It looks more like small, but deep craters shadowing part of their wall. Remember this is a very low resolution view, it's magnified 2x. We used to think wispy terrain on Dione was brighter deposits on the surface, they turned out to be steep cliffs from close up.
The shadowing of the crater walls maybe more visible in my false-color composite earlier in this thread, than the grayscale image above as I sharpened it a bit. Note there's a population of small craters, only they appear dark.
I'm not sure this imaging op will do much better in terms of resolution, either.

[jasedm]

Yes, it's probably wishful thinking on my part - admittedly, it's a relatively low-res shot, and magnified twice, but the phase angle is minuscule (7 degrees) so Janus is almost 'full-moon' here. Furthermore, the dark marks are concentrated on the centre of the best-lit portion (given Janus' lumpy shape) of the visible hemisphere, rather than towards the limb, strongly suggesting albedo features.
I agree, it's unlikely this pass will resolve the issue (I'm not sure of the expected phase angle of any observations), but it would be very interesting if Hyperion had an analogous twin...

[nprev]

Speaking of icy moons, check out this recent shot of Iapetus: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=83496

REALLY odd looking....is this a south polar view?

[Rob Pinnegar]

REALLY odd looking....is this a south polar view?

No, that's the Saturn-facing hemisphere that we usually see. Maybe what's throwing you off is that north is to the left here.

[Decepticon]

Here's another of Iapetus http://saturn.jpl.nasa.gov/multimedia/imag...3/N00066116.jpg

WOW!

[edstrick]

I remain convinced that there a black monolith on the central peak of that crater!... and.. IT'S FULL OF STARS!

[nprev]

I remain convinced that there a black monolith on the central peak of that crater!... and.. IT'S FULL OF STARS!

Yeah, but if you go inside all that happens is that you turn into a baby inside a hotel room...

[ugordan]

Yeah, but if you go inside all that happens is that you turn into a baby inside a hotel room...

Well, here's hoping that doesn't happen to Cassini too, come Sep 10, 2007...