[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.