With a month to go before the F-ring orbits commence, I thought it would be good to open a discussion about what exactly to expect, and the challenges and risks associated with the final months of Cassini's mission.

The final phase has been described as a mission in itself - one that might have attracted funding if it were stand-alone rather than one planned for the end of an already phenomenally successful enterprise.

We will no doubt continue to get regular updates at ciclops a resource which has been invaluable to followers of the mission, and which have provided a brilliant insight into upcoming science activities during each orbit, since early 2007 (Thanks Jason).
I'm hoping too that perhaps Emily may post one of her excellent articles on the TPS blog that give more of an inside track on the science that is planned for the final months.

To summarise, the F-ring orbits commence on November 30th and comprise 20 orbits of the spacecraft with periapses just a few thousand kilometres outside the F-ring - this will allow the opportunity to image the dynamism of the F-ring as never before, as well as (imaging opportunities willing) our best views of the rings and ring-moons - Atlas, Pan, Daphnis, Pandora, Epimetheus and Janus (Prometheus has already had it's closeup) There may be opportunities to get images of some of the known ring clumps (S/2004/S6 if still extant) and/or the known propellers/clumps in the outer A-ring such as S/2009/S1 and Bleriot/Earhart. The rings are simply gigantic though, and many of the orbits of the ring-embedded moons are chaotic so probably I'm hoping for a bit too much.

From what I can gather, there seems to be less risk with the F-ring orbits than with the proximal orbits as Cassini has sampled this environment to a degree already, and I believe it is intrinsically less dusty than the D-ring, and with relatively low radiation exposure to Cassini.

The 23 proximal orbits commence on 23rd April next year following the penultimate Titan flyby, and will thread between the inner rings and Saturn's cloud tops. This is slated as the opportunity to pin down Saturn's rotation, measure the mass of the rings and obtain unparalleled data on Saturn's atmosphere. This is where the spirits of adventure and exploration reach their zenith, as the environment between the rings and the planet is not fully understood.

The attached article gives an insight into the enormous planning challenges that these orbits present. It's fairly technical, but in summary:

The dust and radiation hazards present unknown risks to not only Cassini's science instruments but also the ability of the spacecraft to maintain it's optimal orientation whilst preventing any safing events.
As I understand it, it's not possible to prevent the glare of Saturn's atmosphere and rings blinding the sun sensor and star trackers, as they are located on the HGA which is facing forward to minimise risk of damage to the science instruments, so it's planned to command Cassini to 'suspend' star identification for 5 hours either side of each periapsis - thus flying blind during the 'hairy' part of the orbit.
Mission controllers also have to deal with the not-fully constrained effects of aeroheating during periapses which may affect the instruments, and there is also a risk of safing due to radiation constrained within Saturn's magnetic field.
The article states that the first proximal orbits will 'test the water' as regards the environment in that region, with periapse being lower on the final five orbits.

It seems that many of the 'safing' protocols/thresholds will be relaxed during periapses, hopefully preventing Cassini entering safe mode - perhaps one of the worst-case scenarios - each orbit at this stage is only seven days, which doesn't allow a huge amount of time to upload new commands should that happen.

I can only marvel at the technical brilliance of people involved in Cassini-Huygens and all they do, and have no doubt the final months of the mission will be a fitting end to this generation's exploration of Saturn.

Cassini has just passed the last periapse of its F-ring orbits, in 2 1/2 days its last targeted Titan encounter will shift Cassini onto proximal orbits.

Some links to information and movies:

Animated video about Cassini's Grand Finale https://youtu.be/xrGAQCq9BMU

Information and links from JPL http://saturn.jpl.nasa.gov/grandfinale

First dive Wednesday April 26th.

Less than 150,000 miles out now.

Fingers crossed!

5:19 am EDT

Click to view attachment

I trust that Cassini has successfully made it, but our first chance to know for sure is about 20 hours out. From a recent tweet:

"Stay tuned! Earth's first opportunity to regain contact with @CassiniSaturn no earlier than ~midnight PT April 26 (3am ET, 7am UTC April 27)"

Carrier?
https://eyes.nasa.gov/dsn/dsn.html

Downlink started!

Awesome.

Great news!

Images--see Twitter LINK

Excuse my ignorance, but this hurricane is the one at the North Pole, right?

Saturn's north pole – and those of its satellites – are now in full sunlight nearing mid summer. The south poles are in the dark now, but were in late summer when Cassini arrived.

Yeah, here's a first amateur panorama of the north pole cloud structure.

Yes.

The south pole is currently in the shadows (saturnian winter) and looks like this:

Click to view attachment

I made a gif of the dive!
http://i.imgur.com/i6uMzG2.gifv

Nice colours in this recent view (RGB stack from W00107043, W00107044 and W00107045)

Click to view attachment

Although I'm not quite sure what we're looking at here. Night side of Saturn with some inner rings?

Enceladus spraying away on April 27. Reminds me a lot of the Io volcano footage from Voyager 2.



EDIT: I didn't realize there would be more to this observation! D'oh!

Not sure if this has been posted, but this movie puts the images in context nicely:

NASA: Cassini's First Fantastic Dive Past Saturn

Wildespace said, just above:

"Nice colours in this recent view (RGB stack from W00107043, W00107044 and W00107045)

Although I'm not quite sure what we're looking at here. Night side of Saturn with some inner rings?"


I think probably the outer rings, E at the top and G in the middle in forward-scattered light, with the limb of Saturn below, and a long exposure.

Phil

We're looking at the south pole of Saturn (currently in mid-winter darkness), and the G and E rings. In the background is a star field which straddles the Orion / Taurus border:

Click to view attachment

http://nova.astrometry.net/user_images/1609771#original

Interesting brightness changes on the limb. Is this Saturnian twilight modulated by ring shadows?

With much of the attention on Saturn and the rings, Titan has decided to put on an extra special clouds performance: https://saturnraw.jpl.nasa.gov/multimedia/i...9/N00281060.jpg

The background writeup for the cloud picture.
https://saturn.jpl.nasa.gov/resources/7673/
It's good to know that the spacecraft is still able to monitor events at Titan, even with no more close passes scheduled.

The rings do indeed partially block sunlight from reaching the southern border of what would otherwise be the southern boundary of daylight / seasonal darkness. This is suggested by current telescope views of Saturn as seen from Earth and I just confirmed it with the Solar System Simulator's view of Saturn as seen from the Sun. The SSS view of Saturn as seen from below (attached) shows a U-shape, which matches your intuition. Cassini is seeing the tips of the U in a twilight time of day.

Thank goodness for the simulator, because it would take pages of trigonometry to work that out from first principles.

An interesting consequence is that an observer at the right latitude in Saturn's clouds would see two periods of daylight during this season – morning and afternoon – with a couple hours of "night" during the solar eclipse-by-the-rings around noon.

Thanks JRehling for that informative discussion with the simulated view. In that I can note an inner U and an outer U. Perhaps we're seeing more of the inner U in the Cassini image, and this U is from the sunlight coming through the Cassini division? The center of the limb is thus shadowed by the A ring and the limb edges (in the image) show sunlight coming through the Cassini division.

As seen from the Sun (and Earth) the outer edge of the A ring thus completely misses the planet while the Cassini Division continues to intersect the planetary ellipsoid. We are really close to the widest open possible with the summer solstice coming up May 24. It then follows that light passing through the Cassini Division would never hit the terminator point nearest the pole.

I puzzled over the geometry for a while, trying to simulate, in Photoshop, what the planet looks like behind the rings when it occurred to me that the summer and winter geometries are similar, and the parts of the rings that do and don't cast a shadow on the winter pole are exactly symmetrical with the parts of the rings that do and don't have the planet's shadow cast on them on the summer pole. I've attached an image of Saturn that I took on April 15, which is close to the current situation.

The inner edge of the A ring is slightly in the planet's shadow behind the north pole, but the outer edge is not, so the same situation will apply in the south, where it's harder to visualize what's going on. The Cassini Division will paint a curvy stripe of light on the planet near the winter pole, and that's the inner "U" in the view from below.

Soon .....

Click to view attachment

Nice image from 1st May - Sirius (bleaching out dozens of pixels in the camera) just about to be occulted by the F-ring.

I love the little bonus of Pandora photobombing the shot just to the left.

Still a mission full of unexpected delights.



Click to view attachment

Cassini Grand Finale Saturn Portrait (Annotated Version) (April 13, 2017) by Ian Regan, on Flickr

Breathtaking stuff! The definitive image of a backlit Saturn.

Very nice work.

Thanks Jase!

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)

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

I made a video homage to the Cassini Mission:
https://vimeo.com/217370907

No new posts for two whole months? Where is everybody? :-o

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.

Click to view attachment

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.

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

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.

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.

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.

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:

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.

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.

Alas, the forum gurus are stranded on Mars!

Yes, lots going on. But here's something to think about - a long shot, but skilled video astronomers and/or spectroscopists might give it a go,
and the bigger aperture the better

https://arxiv.org/abs/1708.05036

The Cassini spacecraft will enter Saturn's atmosphere on 15th September 2017. This event may be visible from Earth
as a 'meteor' flash, and entry dynamics simulations and results from observation of
spacecraft entries at Earth are summarized to develop expectations for astronomical observability

Interesting. I don't suppose there's a ghost of a chance of getting HST time for this? Wouldn't require a long observation period, but of course the instrument's time is booked years in advance.

EOM press event schedule https://saturn.jpl.nasa.gov/news/3104/nasa-...ion-activities/

A bit of surprise astrophotography from Cassini: http://saturnraw.jpl.nasa.gov/multimedia/i...1/N00287662.jpg

Click to view attachment

Any idea what this object is?

P.S. It turns out to be part of a lovely panorama of the sky:

Click to view attachment

Full-size: http://ibb.co/cYz7E5
http://www.flickr.com/photos/wildespace/36792380385

Images have also been taken in IR, GR, and BL wavelengths, so I'll trust someone will create a nice, clean colour image.