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.

[Bjorn Jonsson]

Cassini has been an awesome mission that I have been following closely since several years before (!) it was launched. I remember the launch back in 1997 as if it was yesterday and ditto for SOI, the Huygens landing and various other events and mission highlights.

I have mixed feelings about how the missions ended. I really like missions to end like Messenger and Venus Express, i.e. to keep going until the fuel is completely exhausted. On the other hand, ending the mission with an atmospheric entry yields valuable data that could not have been obtained by completely spending all of the fuel.

Ranking planetary missions by success is very difficult (and highly subjective) but it is my opinion that the Cassini-Huygens mission is the second most successful planetary mission ever flown; closely behind Voyager.

[djellison]

i.e. to keep going until the fuel is completely exhausted.

So - there was a non zero chance that they would have run out of fuel before the proximal orbits were finished. Hence the drop dead simple plan that set it up all the way back in April - concluding with that one Titan flyby earlier this week that dropped periapsis into Saturn. It was a very elegant (and safe for planetary protection) mission design.

They really left nothing on the table.

[jasedm]

So - there was a non zero chance that they would have run out of fuel before the proximal orbits were finished. Hence the drop dead simple plan that set it up all the way back in April - concluding with that one Titan flyby earlier this week that dropped periapsis into Saturn. It was a very elegant (and safe for planetary protection) mission design.

They really left nothing on the table.

I agree - the trajectory design and execution for the proximal orbits (as for the whole mission) was beautiful.

Interestingly, this excerpt is from one of the significant events report from SEVEN years ago:

"The results of a study of maneuvers near end of mission were presented at the Mission Planning Forum this week. The topics were the delta-v impact of accidental burn-to-depletions and a possible planned burn-to-depletion near the end of mission. An accidental burn-to-depletion occurs when either insufficient fuel or oxidizer remains to produce the thrust level required during a main engine maneuver. The study looked at the delta-v impact of completing the interrupted maneuver at a later time using hydrazine and the RCS thrusters. A planned burn-to-depletion concept was also discussed in the event that bipropellant remains at the very end of the mission. This burn, if performed, would be done during the last proximal orbit to determine how much usable propellant remained in the tank. The study identified times during the last orbit where the burn could be performed and impact into Saturn's atmosphere would still be assured. There are many issues to consider for both of these concepts and the topic will be studied in more detail when the project looks to plan the later segments of the mission."

I don't know why a planned burn-to-depletion wasn't performed in the end - perhaps to allow for the possibility of a clean-up burn if something went awry with the penultimate/final Titan flybys?

It's the end of an era, and there's a Cassini-sized hole in UMSF.................soon to be filled by; Osiris-Rex, New Horizons at 2014MU69, the JWST, Hayabusa2 and so on...

Lots to look forward to!