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Unmanned Spaceflight.com _ Cassini general discussion and science results _ The unlit side of the rings
Posted by: Bjorn Jonsson Jul 27 2006, 10:56 AM
Cassini has previously imaged the unlit side of the rings but now it is doing extensive observations of the unlit side. For the first time in its mission Cassini is spending a significant amount of time north of the ringplane - earlier it has done so only near periapsis but this orbit is different.
This is a quick RGB composite I did from wide angle images:
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I made no attempts to correct the color - this is probably not very far from the true color of the rings. Large color variations are apparent, the C ring and the Cassini division appear much more bluish than the thicker rings (A and B ). It should be noted that these images were very probably downliked with the 12 -> 8 bit encoding which basically means that dark areas appear too bright in this image because I did not correct for this.
It's also interesting to compare this image to Voyager images:
Voyager 1: http://photojournal.jpl.nasa.gov/catalog/PIA02241
Voyager 2: http://photojournal.jpl.nasa.gov/catalog/PIA01955
Voyager 2: http://photojournal.jpl.nasa.gov/catalog/PIA01389
Posted by: ugordan Jul 27 2006, 11:30 AM
Great composite. I think it's a tad on the too-blue side, but still it's very neat. Apart from the C ring and the Cassini division, the F ring is also notably blue - it shows nicely in your composite. It's a shame the raws were lossy compressed, otherwise this would make for a very crisp image. Good to see the rings again, even from above.
Posted by: ynyralmaen Jul 27 2006, 12:28 PM
Beautiful... one really nice effect that's visible is the "terminator" coincident with Saturn's equator. The atmosphere south of the equator is strongly illuminated by ringshine, but the northern hemisphere significantly less so - neat!
Posted by: tasp Jul 27 2006, 01:41 PM
I recall viewing pictures sent back by Pioneer 11 showing the unlit side of the rings.
It had never occured to me the sun only shown on one side at a time and it was pretty confusing.
I would like to think my general level of comprehension of this stuff has improved a bit . . . .
Posted by: Bjorn Jonsson Jul 29 2006, 12:57 AM
Cassini's view of the unlit side of the rings is remarkably different from the one provided by Voyager 1 back in 1980:
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This 'image' is based on intensity scans of Voyager 1 images (top) and Cassini images. The C ring is at left and the F ring at right. The Voyager 1 intensity scan has been corrected for radial distortion so ring features are at their correct relative locations. The Cassini intensity scan is rather crude and has not been corrected so there is some radial distortion, especially in the outer parts of the ring system (the location of major ring features has not changed). Also, since it is based on the JPG versions of the images, exact relative intensity differences are highly uncertain. Despite this it is obvious that there are large differences between the Voyager 1 and Cassini images. In the Voyager 1 images the C ring and the Cassini division appeared brightest whereas in the Cassini images the outer edge of the A ring is brightest with the Cassini division appearing rather dark and the inner part of the B ring appearing quite bright.
The differences in viewing geometry are that the Voyager 1 images were obtained at low phase angle and a low solar elevation angle. In contrast, for the Cassini images the solar elevation angle is high and the phase angle also high.
Interestingly, the Voyager 2 images of the unlit side appear somewhat similar to the recent Cassini images.
The highly varying appearance of the rings as a function of various geometric factors (phase angle, solar elevation angle etc.) is remarkable.
Posted by: alan Jul 29 2006, 07:45 AM
Speaking of high phase angles, according to Solar System Simulator on September 15 Cassini will be pass through Saturn's shadow while at a distance of 2.163 million miles.
http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=699&vbody=-82&month=9&day=15&year=2006&hour=15&minute=00&fovmul=1&rfov=10&bfov=30&porbs=1&showsc=1
I wonder what observations are planned for this.
Posted by: ugordan Jul 29 2006, 10:30 AM
From an imaging standpoint, it looks like a very nice opportunity for a wide angle mosaic. 3 footprints ought to cover the width of the scene so it wouldn't be too expensive to acquire a multicolor set.
Earth should be visible as well -- if it's not drowned out by forward scattered light from the rings. This will be a very high phase angle situation so it's possible the E and G rings will stand out very nicely. The main rings (at least the C and F rings) might be brilliant as well.
Looking forward to it.
Posted by: Bjorn Jonsson Jul 29 2006, 07:53 PM
The occultation lasts about 15 hours so there should be plenty of time to point the RSP towards Saturn without accidentally pointing it at the Sun. Sure looks like an interesting imaging opportunity.
The following animation showing the occultation starts at 2006-09-15 03:00:00 UTC and spans a period of 24 hours. The field of view is 8 degrees.
sun_occult.avi ( 1.3MB )
: 547
Posted by: djellison Jul 29 2006, 08:40 PM
Don't they use these sorts of obs for occultation measurements - VIMS I'd have thought would do well with it - or is there a large 'keep out' for instrument pointing around the sun.
Doug
Posted by: ugordan Jul 29 2006, 09:12 PM
True, both VIMS and UVIS can stare at the sun. When these sorts of observations are planned precautions can be taken -- such as disabling the ISS subsystem. Accidental sun exposure could be dangerous to ISS cameras if they were in the middle of a planned imaging sequence and the shutter opened, allowing the sun to burn the CCDs.
Posted by: tallbear Jul 30 2006, 01:02 AM
I wonder what observations are planned for this.
[/quote]
There are a number of ORS observations of the entire Ring system at this
very high phase viewing opportunity ... and even a high phase set of
observations of Saturn
There is also an RSS Occ observation of the Rings and Saturn
and a Solar Ring Occ observation as well.
In addition, the RSS 'beam' will pass within 10's of km of Enceladus' S Pole
but with DSS 63 down this will only be observed with Ka band
Posted by: alan Aug 1 2006, 01:57 AM
The rings about a quarter of the way up form the bottom of this image look distorted.
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS22/N00064487.jpg
Posted by: dilo Aug 1 2006, 05:38 AM
Alan, I cannot see the distorsion... exactly, which rings and how they are distorted?
PS: thanks for great unlit side view and the beeatiful animation, Bjorn!
Posted by: helvick Aug 1 2006, 06:04 AM
I can see it but to me it looks as if the rings have quite a rough texture - it's almost as if they were drawn by someone with a fairly heavy brush action.
Posted by: dilo Aug 1 2006, 08:27 AM
Perhaps you're referring to the disuniformities visible as small hue variation is this RGB version (with enhanced saturation):
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Anyway, is something barely visible to me...
EDIT: just posted a http://www.unmannedspaceflight.com/index.php?s=&showtopic=2992&view=findpost&p=63263 showing spokes transit in this region during previous day, the rough texture noted by alan seems to co-rotate with spokes...
I made the following mosaic of 3 images showing the A/F rings with Saturn shadow:
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I like it...
Finally, did someone noticed this very odd image?
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=80270
Image explainations refers to "SATURN-ERING" but I suspect that this bright strip is another piece of main rings, partially eclipsed by saturn shape + shadow... other ideas?
Posted by: ugordan Aug 1 2006, 10:16 AM
I assume you mean geometric distortion, the rings in the upper half being curved less? That's perfectly normal when viewing a ring ansa. It's just an optical illusion.
Posted by: ugordan Aug 1 2006, 10:19 AM
Three words: image compression, noise & non-flatfielded raw images. You're trying to pull way too much out of them. When calibrated they will likely look very smooth (apart from spokes) at this distance.
Posted by: ugordan Aug 1 2006, 10:21 AM
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=80270
Image explainations refers to "SATURN-ERING" but I suspect that this bright strip is another piece of main rings, partially eclipsed by saturn shape + shadow... other ideas?
Enceladus or Mimas (at around 90 deg phase) entering eclipse while the camera is staying pointed at the rings?
EDIT: Geez... apologies for the triple post, I assumed the board sw would automatically merge them when posted on a short timescale.
Posted by: pat Aug 1 2006, 11:42 AM
[quote name='ugordan' date='Aug 1 2006, 11:21 AM' post='63268']
Enceladus or Mimas (at around 90 deg phase) entering eclipse while the camera is staying pointed at the rings?
The streaked satellite is actually Janus. Thats a very long exposure image (100-200 seconds) tracking some point in the ring plane. The raws site is listing the target as E ring probably because thats the target model used for the exposure calculations. The camera command (ioi) files have two entries for target, 'TARGET_DESC' and 'TARGET_NAME' . TARGET_DESC is supposed to describe the object the image exposure was calculated for while TARGET_NAME indicates the object/position that the camera is actually tracking. Often these are the same, but not always. Its the TARGET_DESC keyword that makes it into the description on the JPL raw site.
Posted by: alan Aug 1 2006, 12:53 PM
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Anyway, is something barely visible to me...
[color=#FF0000]EDIT: just posted a http://www.unmannedspaceflight.com/index.php?s=&showtopic=2992&view=findpost&p=63263 showing spokes transit in this region during previous day, the rough texture noted by alan seems to co-rotate with spokes...
Yes, that's what I was referring to
Posted by: dilo Sep 13 2006, 06:51 AM
Between the various images coming down now from unlit side, http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS23/N00065991.jpg partially shaded C-ring view is particularly intriguing, with many stars in the background...
What seems odd to me is the long streak on the right of the center (inside the Saturn shadow)... Very similar to an artificial satellite pass in front a stella field, but we are on Saturn so the most probable explaination must be a cosmic ray. A rare one, because a so long track imply strictly radent trajectory and "slow" energy release on the sensor... 
(we see other CR tracks in this image, but they are shorter as usual).
Note also the Saturn night side barely visible in the very bottom edge... 
Posted by: Rob Pinnegar Sep 15 2006, 07:37 AM
Actually, unless I'm very wrong, that's the inner part of the D Ring, not the C Ring. It is a very nice image.
Posted by: alan Sep 15 2006, 10:06 PM
Get your images stitchers ready
September 15, 2006
(Source: Jet Propulsion Laboratory)
Ring scientists have been waiting for this. Finally, after more than two years orbiting Saturn, the Cassini spacecraft reaches one of the ultimate vantage points. The rings should shine with majesty worthy of the "Jewel of the Solar System."
The event is a solar occultation -- when the sun passes directly behind the planet as Cassini looks on. And this is not just any solar occultation; it's a very long one. The Cassini spacecraft will be right where scientists studying the rings want it: far enough from Saturn to be able to image it all and, more importantly, with the Sun blocked by the planet for 12 hours, long enough to properly map the elusive microscopic particles moving within the extended ring system.
Data collected during this observation might also uncover clues about Enceladus' past behavior and aid mission planners in refining ring hazard models for future ring crossings. Thanks to the slow occultation, images taken during this carefully designed orbit may also uncover new ring structures and, at the very least, capture truly spectacular views of the D, F, G and E rings.
"We are all sort of on pins and needles waiting for the results," says Brad Wallis, Cassini Rings Discipline Scientist. "When you get these kinds of high phase angles, very small particles almost focus the light right at the observer. So these faint rings that are so hard to see are going to be considerably brighter and show us details that are just not possible to see in other viewing conditions. All the space between Enceladus and the G ring is probably going to be pretty well lit up. It's really a unique event.'
http://saturn.jpl.nasa.gov/news/features/feature20060915.cfm
Posted by: dilo Sep 16 2006, 11:25 AM
http://saturn.jpl.nasa.gov/news/features/feature20060915.cfm
Amazing!
Hope they will take also a snapshot of Earth trough A-ring...
Posted by: ugordan Oct 2 2006, 09:56 PM
Nice view:
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Posted by: mchan Oct 3 2006, 03:36 AM
That deserves a wow! Can't figure out the lighting geometry...
Posted by: ugordan Oct 3 2006, 06:48 AM
The viewing geometry is similar to the solar occultation sequence last orbit. Cassini's above the ring plane, the sun is below and located at about upper left position. Everything you see is Saturn's nightside and the unlit rings. Saturn's north is to the top. It's striking that while Saturn's southern hemisphere is lit by light reflected off the rings, even the northern latitudes are lit near the terminator by the light forward scattering through the rings. This illumination is noticeably more bluish, probably the effect of the ring particles preferentially forward scattering shorter wavelengths.
Posted by: Floyd Oct 3 2006, 12:33 PM
Could someone help explain what I'm seeing in these recent images of the F ring. In these three images a wake gets more and more pronounced, and I would think would point to a moon (as in the picture of http://photojournal.jpl.nasa.gov/jpeg/PIA07809.jpg)
Here are the 3 pictures of interest:
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS24/N00067296.jpg
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS24/N00067295.jpg
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS24/N00067294.jpg
I think I would see Prometheus or Pandora if they were in the picture?
Thanks, Floyd
Posted by: remcook Oct 3 2006, 01:23 PM
" (G) NAC image taken on 29 October 2004 showing the multiple strands of the F ring, a streamer of material connecting Prometheus with the F ring, and sheared gaps in the inner strands due to previous apoapse encounters with Prometheus."
from: Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites, porco et al. science 2005.
http://www.sciencemag.org/cgi/content/abstract/307/5713/1226
Posted by: mchan Oct 4 2006, 07:29 AM
D'oh! I was thinking how there could be a terminator across Saturn when the planet shadow cuts across the rings like that...
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