Whether this is a two-year extension or longer I'd still have to go with Titan. There is so much of this world which we don't understand and have only begun to unveil that it begs us to focus our attention on this dynamic world. Some important questions that may only be answered with additional flybys:

- What is the source of methane in the atmosphere?
- Are there any bodies of liquids on the surface?
- Is there active precipitation?
- Is Titan geologically active, i.e. cryvolcanism, tectonism, etc?
- What is the extent of the organics present?
- Is there an ammonia/water mixture under the surface?

Since CASSINI cannot be placed in orbit around ANY of Saturn's moons, an extended tour will need to continue to use Titan to get around the Saturn system.

That being the case, Titan should be the prime focus. A season on Titan lasts more than 7 years and what changes will we see as Titan moves into northern spring and the north polar region begins to see daylight again? What's been going on under the dark polar hood for all that time? (talk about the seven year itch!)

Perhaps the orbit can be cranked in such a way so as to get several closer Iapetus flybys while continuing to monitor the Enceladian plumes?

Craig

All of these moons are wonderful and extraordinary in their own ways...
I would like if somebody could refill Cassini's tanks so that it can stay there for another 50 or so years...

Titan would be the main focus, just because, as belleraphon1 said, it is our best way to get around the Saturnian system, especially as propellent resources dwindle. Outside of Titan, I would like the focus to be Enceladus, where I hope to see 2 or more encounters, preferably a gravity pass over the south pole. If we get four encounters for example, I would like to see three gravity passes (south pole, trailing hemisphere, and anti-Saturnian hemispehre) and one ORS pass (over leading hemisphere). I would like to see one more Iapetus pass, though this is of lesser priority, we don't get that many nice passes of this moon, even from a Voyager-class distance (< 100,000 km). To be honest though, Iapetus is nothing more than Rhea with a few quirks. The other satellites could be treated with additional non-targeted encounters.

Although I wouldn't call this a "main focus," I would like to see the trajectory studied hard for the opportunity to do a pretty close flyby (say around 2,000 km) of one of the itty bitty moons, and to give the flyby a resonably dense set of observations. The closest Cassini will get to any of those things during the primary mission is 10,000 km from Telesto, which happened on October 11. Hyperion looked so bizarre up close, I'd like to see what one or two of the itty bitty ones -- Methone, Pallene, Calypso, Polydeuces, or Helene looks like from that kind of perspective.

--Emily

My questions: could one Iapetus pass be designed to answer all the questions that are left unanswered by the 2007 close encounter? Would high resolution views of eastern Cassini Regio (moat/snowman) do it? And how much fuel would this one encounter require?

Michael

Agree with Emily here.

Also ..... towards the end (and ONLY at the end - risk factors here) of ALL extended mission scenarios, some really close passes over or at the edge of the rings would be really cool. Not sure whether relative velocities would make ISS
imaging impractical. But INMS could taste the ring atmosphere......

Craig

I'd like to heartily second Emily's post above (after all, check out my avatar).

Bart

Along this same vein...... what powers the plumes of Enceladus.... ?
I think it would be VERY interesting to get several extremely close passes of one of the other icy moons (if feasible to do BOTH Enceladus and another moon)
just to compare the mass distribution between the two. Is there something radically different about the mass distribution of Enceladus ?

Craig

Craig

Are you sure? I can't say yet how (or if) the new reference trajectory will affect it but there is a non-targeted flyby of Epimetheus on 12/03/07, with the current figures at: Inbound 6364 km flyby, v = 7.5 km/s, phase = 139 deg.

And an earlier non-targeted flyby of Methone on 6/27/07 isn't too shabby, either (Inbound 12636 km flyby, v = 10.3 km/s, phase = 115 deg.).

I'm not totally sure, but I have a list from 2004 of the nontargeteds that the satellites planners were actually planning observations for, and that's the closest one that's in that list. But my list is from 2004 and may have been supplanted by now. I'd be happy to be wrong!

--Emily

I voted for Iapetus, but realistically we need to look at what sensing capability Cassini has, and also how much Hydrazine it has to do the work. I think it makes a lot of sense to keep Cassini in a high polar orbit so it can still be operational in July 2010 when the rings are next lined up with the Sun. I don't think that we'll learn all that much more about Titan by having another fifteen encounters with it. We might get some more detailed mapping. Possibly we could see some Titalogic feature change, but that's a long shot.

My thought would be to put Cassini into a near polar orbit via a series of encounters with Titan and Iapetus, and increase the perisaturn distance roughly the orbit of Iapetus so as to reduce the number of encounters with moons that they need to use Hydrazine to fine tune.

How about sending it on to flybys of Uranus and Neptune? And Chiron?

Maybe you're right in that SOST isn't planning obs during 53EP (nt), which will occur during the S35 sequence. And the most recent S35/S36 SPVT info I've seen is dated September 21, 2004.

On the other hand, I got my figures for 53EP (nt) from Cassini MP's tour event summary, updated on September 20, 2005. I sure hope SOST takes advantage of this distance, assuming, of course, the new, upcoming reference trajectory preserves (or betters) the ~6,400 km C/A.

Periapse at Iapetus orbit???

Actually, there is quite a bit we could do with additional encounters with Titan, if properly configured. There are large regions of Titan, particularly in the leading hemisphere, we have very little coverage better than a few km/pixel. And given that there is a fairly unique region right there in that longitude span (Xanadu) and intriguing features within, we would be remissed to not try to observe those regions at higher resolution. Not to mention monitoring for seasonal changes as we go into Northern spring and southern fall.

I'm sold that there are ring-related phenomenon that are seasonal and that should be observed near equinox, but I hope the trajectory gods don't get too carried away.

We need Doppler-gravity passes to -in theory- sort out the mass distributions and moments-of-inertia

Gotta side with the apparent majority here...I think that long-term Titan seasonal monitoring combined with possibly opportunistic Enceladus passes would add the most value to the mission.

One thing we probably really need to know is whether Titan has "monsoons" at specific times and places, especially before planning surface missions and/or balloons... ...evidence of abrupt surface alterations would be quite valuable.

Like nprev I ended up with the majorityside with Titan, but also for me Enceladus must be said ot be a close runner up. Both are places whith ongoing dynamic processes and so most important in my book.

I voted for Titan, both because it is interesting and because it is the only satellite able to significantly alter Cassini's trajectory during a flyby so in a way it is an 'automatic' target. However, it is a problem IMHO that in a way there really are *too many* interesting targets: Titan, Enceladus, the rings, Saturn, Iapetus, possibly Hyperion, maybe some of the small satellites plus more.

Regarding Enceladus I'd really like to see additional targeted flybys. The south polar region is the interesting stuff there and it starts entering darkness in 2009 or so. So if possible I'd like to see a very close ORS-focused south polar pass as early in the extended mission as possible. Gravity passes could be done later. This assumes more than one targeted Enceladus flyby during the extended mission.

Titan is a planet in its own right, and the only other body in the solar system having an atmosphere, both in density and composition, resembling anything like the earth. That gives it the obvious vote, plus its requirement for gravity assists. Frankly, I think the ring ethusiasts have been overly pampered. I would like to see as much radar mapping as feasible, but after that I have to vote for as many Iapetus flybys as possible. Nothing, absolutely nothing in the solar system, remotely resemles many of the odd quirks of Iapetus. Also, it's the only major moon in the Solar System that formed far away from the parent planet and the rest of the primordial goop that made up the other satellites. How about a radar pass? Would that bring out details that images couldn't?

A minimum-energy transfer from Saturn to Uranus would take about 50 years, during which time the probability of craft failure would only climb. Neptune would be far worse. During the cruise, no close flybys would take place. Even if Saturn-escape were feasible, this would be a way to minimize Cassini's science return.

Really, the priorities have to be (1) Titan, (2) Enceladus, (3) the rings, (4) Iapetus. Titan needs greater spatial coverage (it seems that even the maximal extended mission will result in gaps in the RADAR mapping). Enceladus needs greater temporal coverage (do the jets vary in their activity over mission timescales?), and will be sufficiently mapped at medium, but not high, resolutions in the primary mission. The rings offer vast spans of territory that is not entirely isotropic, and lots of temporal and spatial coverage could be a plus. Iapetus is a world of perhaps as few as one mystery, and it might be resolved with as few as three (or less) choice images that could be snapped in the remaining primary mission flybys if we're lucky.

If Iapetus is not figured out by the end of the primary mission, it should earn just one more well-targeted flyby at most. Titan, though, merits as many flybys as possible.

I agree. Also, you have to understand the situation with Iapetus -- that moon may embody a mystery, but it's a *frozen* mystery, frozen into Iapetus ever since the events that resulted in its current configuration and appearance. Whereas Titan and Enceladus are both exhibiting dynamic changes that can be studied during the course of the mission, Iapetus' secrets will stay safe and sound for many missions to come.

During the remaining time Cassini has to explore the Saturn system, I'd rather see it explore the dynamic systems available for obervation -- primarily, Titan and Enceladus, although Saturn and the rings also qualify.

If we can't unlock Iapetus' secrets in another two or three good, close flybys, I think we really ought to leave that mystery for later missions. That particular mystery isn't going anywhere. But it's always possible to see something unique in a dynamic system that will unlock a greater set of mysteries, either within the Cassini dataset or in combination with additional datasets collected, before, during and after Cassini.

-the other Doug

*THUD!* That's the sound of my Jaw hitting the floor!

How about if we aim Cassini right at the ridge's center, full throttle - that way we not only get some extreme closeups ala Deep Impact, but we can also force Iapetus to split in half for an unprecented look at a Saturnian moon's interior!

I picked Titan. And I wish they would focus on Radar passes. Wishful thinking on my part.

The 2007 DOY 337 Epimetheus flyby is in the existing sequence for observations.

FYI the ROCKS are largely in the hands of the RINGS folk and since accurate ephemerides were not available during the planning phase, a number of the good viewing opportunities are not in sequences at present. The two Methone close flybys are not yet in any sequences. There is a Janus flyby near the end of the mission at ~14000 km which also has nt yet been incorporated into existing sequences.

However, the upcoming Tour adjustment will tweak all these nontargeted Rock flybys to some TBD extent.

The RINGS folk have requested that some close flybys of the ROCKS be planned in the EM and have prioritized the lsit of Rock Targets. Closer is certainly better, but we really want to have flybys where the Rock is well lit and will give VIMS a good data set. So the minimum range requested is one that where much of the Rock will be well lit and will fill at least 1/2 of the NAC FOV.

Many of the Rock observations other than the CLOSE flybys are being accomplished using 'retargetables' and therefore do not show up in any sequence products until final planning of the sequences is underway. The CLOSE flybys that are not yet in sequences will quite probably be sequenced but the time for those observations needs to be negotiated.

This may not belong in this thread...but ... Here is a listing of the 30 closest Rock flybys in the present Tour..( after mid 2005 ).. This list also shows if the observations of each flyby are included in present sequences ... It does not show if there is a retargetable that is planned for observations of any of the flybys.

These are opportunities to look at 13 new unexplored bodies. Are they all captured ? Asteroids ? Comets ? Are they all leftovers from the formation of Saturn ? There's a lot to be leared from exploring this family of small Saturnian Satellites.

"....but we really want to have flybys where the Rock is well lit ...."

A "simplest" figure-of-merit for a flyby and the best time to take images during a flyby is the number of pixels (or square acr-seconds, whatever) on a disk that are in sunlight. It's far from perfect, but it's a good start.

For VIMS .. the NAC FOV corresponds to about 6x6 VIMS 'pixels'

VIMS wants more than 1 pixel on the lit portion of the body ... more is better...

so far... only Epimetheus, Pandora, and Telesto have been covered at a resolution
that gives VIMS a fairly decent data set.

I think we've got it covered as to what makes a good Rock flyby

1/2 of the NAC FOV sounds just great! Looking forward to those views! Just look at what Pandora looks like at less than that:


And Epimetheus was a real surprise to me too:

--Emily

I just saw David Seal's deltas for the upcoming reference trajectory adjustment. For the December 3, 2007, Epimetheus non-targeted flyby (53EP (nt)), it looks as if the 6,364 km C/A is not going to hold. Seal's "Option 1" shows a C/A altitude raise of ~10,828 km; "Option 2" shows a smaller increase of ~3,128 km; and "Option 3" shows an even smaller increase of ~2,169 km. In other words, 53EP (nt) C/A will be 6,364 km + whatever Option is selected.

I haven't looked at the other "Rocky" flybys yet. Maybe some C/A's will drop to yield a relatively close flyby.

I voted for Titan also, on the grounds that atmospheres generally are more interesting than rock and that Titan can swing Cassini around Saturn's moon system. However I'm sure there's plenty of Saturn science that can and should be done from such orbits also and any number of encounters with other moons that can be included in a program. So I would hope and expect that an extended mission would be as rich and varied as the primary mission.

Does anyone know how the propellant stocks are holding up? Following MRO's dramatic saving in fuel on its insertion, I was wondering how well Cassini is performing against budget. For the 18 Mar flyby for example I noticed that one of the targetting manouevres was cancelled. And, are there any other resources we're going to run short of before propellant? The RTG should be good for a long while yet... anything else we should worry about?

The only thing that scares me in the Saturn system is a stray fragment from the rings hitting Cassini.

Well that's one way the mission might end; but the successful ring crossing at SOI suggests that the risk isn't that high, if we consider the next four years on orbits similar to Cassini's post SOI. I was wondering if anyone had an insight into what other degradation or outright loss might be the highest risks in that sort of timeframe.

I think the amount of bipropellant left for the main engine is THE biggest issue. The maneuvering thrusters still have a lot of monopropellant left. The s/c team doesn't publicly release the amount of consumables left, but there was a report here about remaining states and predictions at the end od primary mission.
Of course, when you're dealing with a spacecraft that has any moving parts, those can also fail randomly. We might get a stuck filter wheel on ISS, failure of another of the reaction wheels (which wouldn't be catastrophic, though) etc.
It would be really weird if the RTGs started to fai, failure of the HGA is probably also unlikely as it's bound to be redundant.

Fuel really seems to be the only thing keeping this beast back.

I voted for Titan. A while back, I remember reading about an extended mission option that could brake Cassini into orbit around Titan. I realize that this would require a lot of leftover fuel, and would severely curtail whatever other observations in the system could be done (hence, making it an unlikely possibility), but if it were possible to improve RADAR coverage of Titan Magellan-style, I think it would be worth it--especially for any future landing/balloon missions. Just my two bits.

Robert Mitchell (Cassini project manager) told me flat-out a few months ago that that is just not practical, even with aerobraking -- it would involve far too much propulsive maneuvering by Cassini.

Aerobraking itself is not even practical since Cassini is not designed for it, propellant margins notwithstanding. In fact, analyses performed by Cassini AACS SCO for low altitude (950 km) Titan passes showed that both the MAG boom and RPWS antenna will fail at certain atmospheric densities (p). For those who are gluttons for details, I seem to recall that the RPWS antenna will fail first at p equal to or greater than 1.2 x 10^-7 kg/m^3 and the MAG boom second at p equal to or greater than 2.3 x 10^-7 kg/m^3.

Yeah, and even if the booms didn't break, they'd almost certainly flip the craft during aerobraking. I never thought of that (pause for misguided snickers from Alex).

Cassini will end its primary mission in a highly inclined polar orbit around Saturn -- 80 degrees -- which can nevertheless intercept Titan once at the apoapsis of each of its orbits, and which has a periapsis that's considerably lower. How well could they make protracted studies of Titan, Enceladus AND the rings from just such an orbit? And, by lengthening its apoapsis with gravity-assist flybys of Titan, they could also use it to make more flybys of the other icy moons -- including Iapetus.

This line of thinking reminds me of an idea that I had a while back. I know that there is not enough fuel for Cassini to enter orbit around Titan, but what about entering Titan's orbit around Saturn instead? Especially since the orbit will be (near-)retrograde at the end of the mission, this would allow roughly two Titan passes every sixteen days. If you could afford another 180-degree transfer, you would have fewer passes, but at nice slow speeds. Is a Titan-like Saturn orbit approachable with the remaining delta-V?

Bart

I think the problem is that kind of orbit is not going to be stable. Every time you flew by Titan, the orbit would change considerably and it would no longer be "Titan-like". It would require active corrections to maintain and since Titan's mass is quite considerable, those corrections would need to be very big.

Yea, verily. Lo, on the seventh day of extended mission planning many wonderful things were created, among them a pi-transfer descent option - classes PF, QF, PB, QB, PX, and QX. Beware, though, the false prophets who promise many things, including the IX-1 Tour with 56 non-targeted icy flybys, which includes 24 n/t's of Enceladus with 8 under 50,000 km.

That's only one problem. Does anyone have any good ideas on how Cassini's orbit would be "circularized" to become "Titan-like"? And I don't think the 174 m/s of bi-prop delta-V estimated to available for EM (at 95% confidence levels) would cut it.

I was gonna say repeated Titan flybys probably would be sufficient, but when I think about it twice, I'm not that sure.
As to how long it would actually take to circularize the orbit via Titan flybys (which is a moot point anyway because the orbit would only remain circular till the next Titan close flyby), I don't know.

Crash it into Titan.

Wouldn't it be possible to set up the flybys so that the 'disturbance' of Titan was mostly focused into an inclination change? I'm sure the MAPS guys would love that anyway.

Also, if Titan gravity assist maneuvers can move the apoapsis from Iapetus' orbit to Titan's and back in a few orbits, surely it can't be that hard to adjust the periapsis from Mimas' orbit to Titan's?

Bart

Surprising, but...

Orbital velocity (for a circular orbit) varies with the sqrt(distance). Moving an orbit *in* can take a lot of work, whereas moving it out the same distance will always take less, and the magnitude of this effect can be extreme.

Let's say you have a rocket in solar orbit corresponding to the Earth's orbit. Moving its perihelion to intersect Mercury's orbit will take more delta-v than moving the aphelion to intersect Pluto's.

As a guideline, orbital velocity for a given circular orbit is about 71% of escape velocity. The 71% to get it down to zero is a lot bigger than the 29% needed to get to infinity. So Titan is a lot "closer" to Iapetus than it is to Mimas in energy terms.

It all depends on one's definition of "that hard." Deterministic delta-V is one of the prime limiting factors. Of course, it goes without saying that science objectives (e.g., desirability for icy flybys, occs, etc.) are also in the trade space.

BTW, one really interesting if not wild EM idea I heard was a suggestion to jettison the main engine cover and use it as an impactor (à la Deep Impact).

I just looked up the formulas and did the calculations myself and it looks like a circularizing a Titan/Mimas osculating orbit takes a delta-V of 2.7 km/s, whereas converting a Titan/Mimas osculating orbit to an Iapetus/Mimas osculating orbit takes 3.6 km/s. Obviously, these orbits are oversimplifications, but I think it illustrates the point that circularizing the orbit is not that hard.

As far as where to get that delta-V, the theoretical max that you could get from a Titan-assist is 11 km/s, but I know they're not actually using anywhere near that much.

As an aside, surely someone has compiled the delta-V from each Cassini-Titan flyby somewhere? This would be a really neat addition to one of the 'Saturn Tour' pages like the one on the Cassini homepage, or Emily's much better one.

Bart

EDIT: Ok, I just looked at Emily's page to get a rough idea of the gravity assist magnitudes for the mission so far and it looks like most of them are in the range of 200-600 m/s. This means it would take several passes to circularize the orbit, but probably less than 10.

It would be hard to predict just where it would hit, wouldn't it? (And of course you wouldn't want to do it until you were done using the engine.)


What the periapse of the final orbit planned during the Primary Mission, and how does it compare to the orbital distances of Mimas and Enceladus? (I could look this up, but I'm too lazy.)