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Unmanned _ Cassini's ongoing mission and raw images _ The Grand Finale

Posted by: jasedm Oct 27 2016, 01:34 PM

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 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 biggrin.gif 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 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.

Posted by: alan Apr 19 2017, 07:15 PM

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.

Posted by: Floyd Apr 21 2017, 01:19 PM

Some links to information and movies:

Animated video about Cassini's Grand Finale

Information and links from JPL

First dive Wednesday April 26th.

Posted by: MahFL Apr 26 2017, 04:16 AM

Less than 150,000 miles out now.

Posted by: jasedm Apr 26 2017, 05:48 AM

Fingers crossed!

Posted by: craigmcg Apr 26 2017, 09:23 AM

5:19 am EDT

Posted by: Floyd Apr 26 2017, 02:35 PM

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 no earlier than ~midnight PT April 26 (3am ET, 7am UTC April 27)"

Posted by: Ames Apr 27 2017, 06:28 AM


Posted by: Explorer1 Apr 27 2017, 07:20 AM

Downlink started!

Posted by: MahFL Apr 27 2017, 08:50 AM


Posted by: jasedm Apr 27 2017, 10:03 AM

Great news!

Posted by: Floyd Apr 27 2017, 11:44 AM

Images--see Twitter

Posted by: B Bernatchez Apr 27 2017, 04:40 PM

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

Posted by: JRehling Apr 27 2017, 05:31 PM

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.

Posted by: SigurRosFan Apr 27 2017, 06:38 PM

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

Posted by: wildespace Apr 28 2017, 03:01 AM

QUOTE (B Bernatchez @ Apr 27 2017, 05:40 PM) *
Excuse my ignorance, but this hurricane is the one at the North Pole, right?


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

Posted by: avisolo Apr 28 2017, 09:17 AM

I made a gif of the dive!

Posted by: wildespace Apr 29 2017, 09:46 AM

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?

Posted by: Astroboy Apr 29 2017, 11:46 PM

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!

Posted by: MichaelJWP May 4 2017, 10:20 AM

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

Posted by: Phil Stooke May 4 2017, 01:47 PM

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.


Posted by: Ian R May 4 2017, 03:41 PM

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:

Posted by: scalbers May 4 2017, 04:07 PM

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

Posted by: ngunn May 9 2017, 01:21 PM

With much of the attention on Saturn and the rings, Titan has decided to put on an extra special clouds performance:

Posted by: tanjent May 10 2017, 12:17 AM

The background writeup for the cloud picture.
It's good to know that the spacecraft is still able to monitor events at Titan, even with no more close passes scheduled.

Posted by: JRehling May 10 2017, 03:42 PM

QUOTE (scalbers @ May 4 2017, 09:07 AM) *
Interesting brightness changes on the limb. Is this Saturnian twilight modulated by ring shadows?

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.


Posted by: scalbers May 10 2017, 11:05 PM

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.

Posted by: JRehling May 11 2017, 02:44 AM

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.


Posted by: Ian R May 13 2017, 12:26 AM

Soon ..... cool.gif

Posted by: jasedm May 14 2017, 08:00 PM

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.


Posted by: Ian R May 15 2017, 05:48 AM by, on Flickr

Posted by: jasedm May 15 2017, 05:38 PM

Breathtaking stuff! The definitive image of a backlit Saturn.

Very nice work.

Posted by: Ian R May 20 2017, 03:48 AM

Thanks Jase! cool.gif

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

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