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The Grand Finale, Proximal orbits
JRehling
post Sep 15 2017, 09:27 PM
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In reference to past missions, and what we've lived through, it is perhaps obvious but must be stated in respect to the greatness of this board and those who run it…

The first big mission I was old enough to remember was Viking. And Viking was a masterpiece – four craft exploring the heck out of the most interesting planet and one really can't imagine the technology of the time doing it any better.

But my main source of Viking info was National Geographic and "year books" that supplemented encyclopedias. Occasionally, something would make the print or TV news, but the information cycle for me was more typically one year. Then a book summarizing the main mission might make it into my hands. Viking happened "while" I was alive and waiting for the results, but it wasn't all that different from reading about something that had happened in the 18th century. There was a before and an after, but no during. Likewise Pioneer Venus and Voyager at Jupiter. By the time of Voyager at Saturn, I was subscribed to publications that shortened the news cycle considerably, but it was rarely under a week.

In sharp contrast to that, I remember chatting live here about the first ISS pictures of Titan and manipulating them in Photoshop and having the real sense that I might be seeing something before anyone else ever had… and if I weren't first, I was certainly no more than minutes behind whoever was. Then, when Huygens arrived, a day when I shamelessly did no work at work, the fact that the first image was one of drainage channels in the headlands – unbelievable! And the descent more or less happened here in real time (plus some speed-of-light, downlink, etc. delay), with commentary (and sometimes it's well that interested amateurs are game to speculate more than a publishing scientist should). Viking happened in my lifetime, but Cassini and Huygens happened and were experienced here. And for the life that this provides the flow of information, I am tremendously grateful.
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Bjorn Jonsson
post Sep 16 2017, 12:08 AM
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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.
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Keatah
post Sep 16 2017, 12:52 AM
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QUOTE (Bjorn Jonsson @ Sep 16 2017, 12:08 AM) *
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.


They essentially did just that. Down to 1 or 2 percent fuel remaining. They probably could have gotten a few more Revs, but definitely not enough to start a new science campaign or segment/mission.

This Grand Finale should complement what Juno is doing at Jupiter. I'm surprised that so few people see (or don't bother to mention) the added value from such a comparison between the two datasets.
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J.J.
post Sep 16 2017, 01:02 AM
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Congratulations to everyone on the Cassini team for bringing such a marvelous and inspiring mission to the public!


--------------------
Mayor: Er, Master Betty, what is the Evil Council's plan?

Master Betty: Nyah. Haha. It is EVIL, it is so EVIL. It is a bad, bad plan, which will hurt many... people... who are good. I think it's great that it's so bad.

-Kung Pow: Enter the Fist
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Bill Harris
post Sep 16 2017, 01:08 AM
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I wouldn't rank missions. Each mission returns data and is a success that stands on its own. And each mission serves as a basis or a foundation for future missions.

--Bill
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djellison
post Sep 16 2017, 01:38 AM
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QUOTE (Bjorn Jonsson @ Sep 15 2017, 05:08 PM) *
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.
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NMRguy
post Sep 16 2017, 03:56 AM
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Many thanks to the science and engineering teams that designed, delivered and kept this mission running so well for so long.

Cassini-Huygens was truly a mission of discovery. Cassini's Jupiter flyby delivered the full promise that Galileo could have provided, and then the Saturn primary mission yielded exhilarating exploration, including the early Phoebe encounter, Saturn orbit insertion, Huygens landing on Titan, radar and VIMS imaging of the Titan surface, Enceladus geysers, the Iapetus equatorial mountain ridge, and the Grand Finale Juno-like investigation of the giant planet, just to name a few.

While we all knew this day would come, it is a bittersweet passing onto a different, post-Cassini era.
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marsbug
post Sep 16 2017, 08:37 AM
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The last 13 years have been amazing - it's been a great honour to be alive to witness them - and I'm sure there are many more discoveries lurking in the archived Cassini data. My heartfelt thanks to everyone who worked on the mission. May future exploration efforts do its memory justice.


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Astroboy
post Sep 16 2017, 05:30 PM
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As an archivist, history buff and a fan of vintage electronics, I'm mainly interested in the older missions, i.e. anything that carried a vidicon and a tape recorder and became one of the first couple spacecraft to encounter a specific planet or moon. As a 23 year old, I never got to experience any of them in real time (besides Voyager's interstellar journey), which saddens me. However, when I read recollections such as JRehling's - that for the public during the '70s, there was often just a before and an after, with no live experience like you'd get with a '60s Apollo mission - I am brought back to stark reality.

Cassini though? Cassini felt like an idealized version of those formative missions, and that has caused me to grow very fond of it and regard it as one of my favorite modern missions. It was like being able to follow, in real time, a Mariner or a Voyager that lasted a really long time, never chronically malfunctioned, and had a perfect suite of instruments, including some successors to Voyager hardware (like RPWS). Every flyby of a moon and every revolution in general felt like an event - like a Mariner or Voyager flyby. Having access to stuff like NASA's Eyes and being able to see the raw images as they came down really enhanced that "event" feeling. I think Cassini is as much of a successor to those old missions as New Horizons is.

All things must come to an end, though, and this was the perfect ending. The death plunge kept it from slowly dying and then becoming paper weight in a derelict orbit - it performed flawlessly until the very end, it became a part of Saturn in the process, and some interesting atmospheric science will probably come out of it. Many, many thanks and congratulations to all the champs on the Cassini team, and like others, I am very grateful for their openness to the public that allowed us to experience the mission as it happened. I hope someday we'll get to see a successor to this magnificent machine at Uranus or Neptune.


--------------------
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Gladstoner
post Sep 17 2017, 05:09 AM
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It may have been mentioned already, but Cassini's entry marks the point when all five classical planets have been directly touched by humanity. I wonder what the ancient astronomers would think.
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jasedm
post Sep 17 2017, 11:06 PM
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QUOTE (djellison @ Sep 16 2017, 02:38 AM) *
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!
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toddbronco2
post Sep 18 2017, 01:55 AM
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The Prop team wanted to perform a "burn to depletion" all the way until the end of the mission, but that was vetoed years ago. The logic at the time was that it wasn't worth the risk to the INMS plunge science to perform the engineering test. There was also some nebulous concern about instrument contamination. If you burn to depletion then you will run out of either fuel first or oxidizer first and whichever one is left will then vent unburned as a cold gas thruster. There was some concern that the unburned fuel or oxidizer might coat or contaminate the science instruments.
There were a whole bunch of engineering tests that were seriously and jokingly suggested: jettisoning the ME cover to test if the unused pyros still functioned, performing a burn with the unused backup Main Engine, etc
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JRehling
post Sep 18 2017, 03:13 AM
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Some calendrical bits of trivia that I find jarring: Cassini was launched only 11 months after the launch of Mars Global Surveyor, three years after the end of Magellan, and less than 2 years after the arrival of Galileo at Jupiter. The first pair is perhaps most significant, because Cassini was, in some estimations, the last hurrah for big "Battlestar Galactica" missions and MGS was an early exemplar in the faster, better, cheaper (FBC) paradigm that the failure of Mars Observer, a sort of Cassini-for-Mars, ushered in.

I can't help but think of how unfortunate it was that all of Cassini-Huygens' discoveries came too late for Carl Sagan to enjoy them. The initial reconnoissance of the solar system's major worlds was nearly complete when he died in 1996, but when one thinks of Titan and Pluto, it feels as though the solar system kept much of the best for last.
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stevesliva
post Sep 18 2017, 02:15 PM
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QUOTE (JRehling @ Sep 17 2017, 10:13 PM) *
because Cassini was, in some estimations, the last hurrah for big "Battlestar Galactica" missions and MGS was an early exemplar in the faster, better, cheaper (FBC) paradigm that the failure of Mars Observer, a sort of Cassini-for-Mars, ushered in.


The mention upthread of just-in-case-pyros for the main engine cover and the SPARE main engine had me chuckling about the battlestarness of Cassini.
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jasedm
post Sep 18 2017, 06:47 PM
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QUOTE (toddbronco2 @ Sep 18 2017, 02:55 AM) *
The Prop team wanted to perform a "burn to depletion" all the way until the end of the mission, but that was vetoed years ago.


Thanks Todd for the insights.

It would be interesting to hear some more of the engineering tests that were vetoed (crazy or otherwise) biggrin.gif biggrin.gif
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