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The Grand Finale, Proximal orbits
Ian R
post May 15 2017, 05:48 AM
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Cassini Grand Finale Saturn Portrait (Annotated Version) (April 13, 2017)
by Ian Regan, on Flickr


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jasedm
post May 15 2017, 05:38 PM
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Breathtaking stuff! The definitive image of a backlit Saturn.

Very nice work.
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Ian R
post May 20 2017, 03:48 AM
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Thanks Jase! cool.gif

The other version of the mosaic (sans annotations) is actually presented 'upside-down', which is just an aesthetic choice:


Cassini's 'Grand Finale' Saturn Portrait (April 13, 2017)


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Holder of the Tw...
post May 31 2017, 02:20 PM
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Cassini survived a close brush by the D ring, no word yet on whether it encountered much in the way of particle impacts.

Spaceflight Now article
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avisolo
post Jun 8 2017, 09:46 AM
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I made a video homage to the Cassini Mission:
https://vimeo.com/217370907
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wildespace
post Aug 8 2017, 09:41 AM
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No new posts for two whole months? Where is everybody? :-o


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Phil Stooke
post Aug 8 2017, 08:05 PM
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Too much other stuff going on! But I feel your pain so how about this?

Two images of Dione taken on August 1st, as part of a long sequence of Dione and Enceladus shots. The right image shows a very narrow sunlit crescent at bottom, and a much wider area lit by sunlight reflected off Saturn. Bear in mind that sunlight on Saturn is about 1 percent of its intensity at Earth, so the night side of Dione is illuminated by a weak light source. The left image is a long exposure shot of the same view, where even the very faint illumination at the 'limb'* of the Saturn-illuminated area is overexposed. The bright disk is surrounded by a fuzzy halo. Normally I would associate that with light from the overexposed region scattered in the camera optics.

But... but... the unilluminated crescent of Dione appears to be silhouetted against that scattered light. I'm trying to think of a way for that to happen without some of the light being scattered by a faint cloud of material around and behind Dione. No luck so far. So is this a real 'haze' around the moon? I don't know.

Phil

EDIT: * I mean, of course, the limb as seen from Saturn, the terminator of Saturnshine as seen from the spacecraft.

Attached Image


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ngunn
post Aug 8 2017, 10:34 PM
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I noticed this and assumed that Dione was orbiting within a very diffuse 'atmosphere' similar to Enceladus within the E-ring. Not knowing if this was to be expected I didn't remark on it, but maybe this is a new observation.

(By the way, we are all still here!)

On another subject, the recent images of Titan's lakes and seas have been superb and I hope some of our imagers will work their magic with those.
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john_s
post Aug 8 2017, 10:38 PM
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I noticed that too. One other possibility is that it's a double reflection: sunlight scattered off the sunlit side of Dione and then off ambient E-ring particles close enough to Dione to be illuminated by it, but not necessarily concentrated near Dione. But that would require the backscattered illumination of the E-ring particles by Dione-light to be comparable to the forward-scattered sunlight from the same particles, which may not make sense.

John
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ynyralmaen
post Aug 8 2017, 11:09 PM
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Depending on the path between Cassini and Dione through the E ring, maybe this fuzzy halo is due to forward scattering of the light from Dione by E ring grains that were between the moon and spacecraft?

Alternatively, Dione does have a tenuous exosphere which could be considered a potential cause, but I suspect that it'd be surprising for it to be this bright. The falloff in brightness could be compared to the expected scale length of the exosphere. I can't see any hint of a shadow being cast opposite to the narrow sunlit crescent, which I think may be expected if the halo is due to material, either gaseous or particulate, concentrated around Dione itself.
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jasedm
post Aug 9 2017, 02:04 PM
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QUOTE (wildespace @ Aug 8 2017, 10:41 AM) *
No new posts for two whole months? Where is everybody? :-o


I think we're all used to seeing and discussing spectacular images, and this last phase of the mission focuses more on fields and particles, rather than the ISS.
Not that there aren't some fantastic images of the rings coming down, but I think Cassini's cameras are not able to take close-up un-smeared images of the cloud-tops in the way that Juno can, and in any case, other instruments are 'prime' at closest approach.

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fredk
post Aug 9 2017, 03:16 PM
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QUOTE (john_s @ Aug 8 2017, 11:38 PM) *
But that would require the backscattered illumination of the E-ring particles by Dione-light to be comparable to the forward-scattered sunlight from the same particles

Why do you say that? Do we know what the intensity of the forward-scattered sunlight is? Do you think there's a background glow away from Dione that's due to forward-scattered sunlight? Wouldn't it be hard to tell if such a background was due instead to backscattered light from the E ring?

The glow appears to me to be consistent with backscattered illumination of E-ring material from the sunlit side of Dione, which would be directed away from the camera and slightly towards the 7 o'clock direction. Forward scattering of the light from Dione by E-ring grains that were between the moon and spacecraft seems unlikely since the dark crescent is much darker than the adjacent glow.
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GS_Brazil
post Aug 9 2017, 07:12 PM
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QUOTE (wildespace @ Aug 8 2017, 06:41 AM) *
No new posts for two whole months? Where is everybody? :-o


And how about this image:



I personally think this is one of the most amazing images of the ring I've ever seen.
If I'm interpreting the image well this is similar to these roofing sheets:

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ynyralmaen
post Aug 9 2017, 08:19 PM
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QUOTE (fredk @ Aug 9 2017, 04:16 PM) *
The glow appears to me to be consistent with backscattered illumination of E-ring material from the sunlit side of Dione, which would be directed away from the camera and slightly towards the 7 o'clock direction. Forward scattering of the light from Dione by E-ring grains that were between the moon and spacecraft seems unlikely since the dark crescent is much darker than the adjacent glow.


I was thinking that as the thin crescent as seen from Cassini is fully sunlit, it may be that there are more forward-scattering particles between the sunlit crescent and Cassini that there are relatively near Dione to backscatter sunlight from the daylit side as a whole. Forward scattering around the bright crescent could also explain the apparent enhanced brightness around the 7 o'clock position.

However, your point about the unilluminated portion of Dione being darker than the surrounding glow is a very good one, so yes, the source must be at least partially behind the moon.
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jasedm
post Aug 9 2017, 09:21 PM
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QUOTE (GS_Brazil @ Aug 9 2017, 08:12 PM) *
I personally think this is one of the most amazing images of the ring I've ever seen.
If I'm interpreting the image well this is similar to these roofing sheets:



Given that the rings are reckoned to be only a few hundred metres (a kilometre perhaps) thick, my guess is that this is an optical illusion. The rings are very open currently (around 27 degrees with respect to the earth) and I'd be surprised if shadows would be cast at this angle of incidence. Different if this image had been taken around equinox of course.

Nevertheless amazingly intricate and beautiful.
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