The Grand Finale, Proximal orbits Options

[jasedm]

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.

[JRehling]

This juncture might be a time to remember that extended mission concepts once proposed for Cassini included going into Titan orbit or leaving Saturn orbit and journeying to Uranus. It sounded like the first was infeasible and the second simply unwise.

[Explorer1]

Yep, I remember the idea of Cassini ending up in Titan orbit from both a Stephen Baxter novel, and some speculation on how it could be accomplished in this very forum, circa 2004! ( http://www.unmannedspaceflight.com/index.php?showtopic=559 ). I still do wonder how plausible the main antenna as a heat shield would be...

[stevesliva]

leaving Saturn orbit and journeying to Uranus

I think the Saturn orbits >>> Uranus flyby.

If we don't get an ice giant orbiter, though, we damn well better get a couple flybys!

[JRehling]

I don't think the Uranus option ever made much sense. It would have cost a lot of Saturn science and created a huge and risky delay before arrival at Uranus. If anything, it was more of an impressive statement of possibilities.

Of course, in some alternate reality, alternate versions of us in 2029 will say, "Wow, if Cassini hadn't been sent to Uranus, we never would have known about _______!"

[jasedm]

I don't think the Uranus option ever made much sense. It would have cost a lot of Saturn science and created a huge and risky delay before arrival at Uranus. If anything, it was more of an impressive statement of possibilities.

Agreed.

I think the right decision was made - a 'free' atmosphere probe at Saturn (although not optimised as such) will surely pay dividends. With Uranus' axial tilt also, flyby science would have been shoehorned into the relatively brief period a couple of days either side of closest approach.

It's more than a pity however that an almost perfectly-operational spacecraft had to be disposed of for want of fuel. As ever, it's all in the delta V....

[Explorer1]

During the finale coverage, one member of the team (I forgot who) said that another possible alternative for disposal would have been an orbit far beyond both Enceladus and Titan. It may have been possible, but all the good science is of course in the centre of the system, so they decided to follow the science.

Sure, a few flybys of the irregular moons would have be good, along with plasma and dust science, but they balanced that against the ring and atmosphere science (we'll see what is recovered from the final dive)

See slide 19 here: http://www.lpi.usra.edu/opag/march_08_meet...ons/spilker.pdf

[atomoid]

gif anim of the parting Enceladus-set from September 13

[Ian R]

My pictorial tribute to our dear and departed robotic friend ....


Cassini's Last Dance — A Final Portrait of Saturn by Ian Regan, on Flickr

[Explorer1]

The CICLOPS team version of the final mosaic: http://ciclops.org/view/8631/A-Farewell-to-Saturn

Farewell...