I agree with ugordan, scientific goals should drive the mission and instrumentation. And I fall into the camp of desiring to study one object thoroughly in order to extend those lessons to others.

So which Galilean moon will tell you the most about the formation and evolution of common classes of planetary objects?

Studying Europa will tell us about watery moons with thin icy shells in special environments. Very interesting, but very specialized. Basically Europa would only serve as a model for itself and Enceladus.

Studying Ganymede will tell us about big ice moons/planets with thicker crusts. Titan, Triton, Pluto, and others may all fall into this category.

Ganymede seems a more general "Joe-big-ice-world", while Europa is more of a special case. Both are very interesting cases, but Ganymede's lessons will extend to more objects.

-Mike

I'm not sure that all of the objects have a system-wide quantum leap in them. I'll point the finger at Callisto first: Aside from trace compositional differences due to the higher radiation flux smacking its trailing side, it pretty much looks the same all over. Sure, a mega-mission with landers and rover would generate new Callisto science ad infinitum, but orbital surveys may not be able to do much more than either JSO or EE already have planned.

I wouldn't bet the farm that Ganymede is in the realm of "the more you look, the more new stuff you see". Again, taking into account that EE will visit Ganymede more than any Cassini will visit any saturnian satellite save Titan (a doubly extended mission may push Enceladus to that number of flybys), I can't presuppose that there's a quantum leap there that we'd be missing. Fourteen ground tracks at low altitude makes for a lot of sampling that surface. The old stuff is probably not more diverse than Callisto. And the groovy stuff would get a lot of close peeks from 14 flybys. Don't pigeonhole EE as a mission that ignores the rest of the system.

Io almost certainly would turn up more diversity the more closely we looked at it. JSO would do a so-so job of that. I would lean more towards EE now and a later mission that flew a quasi-Juno-like orbit chock full of Io flybys so long as it would end up observing the whole surface in daylight at some point or another. Although the nice thing about Io's plumes is that you can actually make useful observations at night, too.

The thing that makes Europa incredibly interesting to me I could sum up with this question: How old is the youngest crust? I can tell you the answer to that for all of the other Galileans right now: Zero age (Io) and more than two billion years (Ganymede and Callisto). Europa... could be crust formed today, could be sixty years old, could be ten thousand. And if the answer is on the low end, we know that it is local (the majority of the surface is old enough to show sporadic impact cratering), and it could be arbitrarily local. We won't know about the third-next mission to this system (Juno, JSO/EE, a possible Europa lander) until EE flies. That's why to me the answer is to fly EE, get a heck of a good look at Ganymede (without making it the best mapped world in the cosmos), and aim to give Io its closeup later with a mission designed for Io.

As a final pessimistic comment on remote sensing in this system, I'll point out that IR studies of composition have utterly struck out when done at the Galileo and MGS-TES level. I'd want to see a heck of a good probability of an answer before flying a mission with a high priority on getting compositional data and getting back answers like Galileo re: Ganymede (it's something dark mixed in with ice) or MGS (there are two kinds of rock under the dust, and one might resemble andesite but we're not sure).

Looks more & more like we're not going to reach a consensus here, which is interesting. It's very clear now that there are numerous science objectives of great interest in the Jovian system, and one mission probably won't cut it even for the questions we know to ask, much less the ones we don't know yet.

Redirecting a bit if I may, should OPAG then derive an integrated mission campaign for the Jovian system as a whole, similar to that for Mars? Understand that we can't do the same launch rate, but perhaps a NASA/ESA/JAXA collaborative planning effort could cover all the essential bases over a reasonable time-frame. We could do considerably more working in parallel, especially right now when we have both specific Europa objectives and a number of things to find out about the other moons.

Just to totally screw things - I'd rather we went back to Titan than any of the Jovians...but maybe that's just me

Doug

Yeah...that'd do it, all right!

LOL Yeah, that's a good point to keep in mind, we aren't just talking about EE and JSO, but Enceladus and Titan as well. As I said before, I don't think we can make a decision on whether Enceladus needs a flagship mission just yet. I think by the end of the Cassini mission, the question of the source of the jets will largely be answered. If they come almost directly from a liquid water source, then Enceladus jumps ahead of Europa quite frankly. Why send 2 or 3 missions to sample potential life when you can send one If the jets do come from a liquid water source, we have the trifecta: liquid water, organic molecules, and an energy source, and the reservoir can be sampled MUCH more easily than at Europa (assuming we can capture this material on a Stardust-like pass and bring it back to Earth). If the jets don't come from a liquid water source, then frankly, Cassini's coverage at Enceladus should be quite sufficient by the end of the extended mission.

As for Titan, I think the kind of science that can be done at that one target would be far more exciting, frankly, than Europa. Not as diverse as JSO, though. Hense my earlier ranking of: 1) JSO, 2) Titan Explorer, 3) Europa. Enceladus could be #1 if the jets come from a liquid water source, #4 if they don't.

But that's, as you would say, jumping the queue.

I've scanned the Enceladus report and was surprised to find how good a case they managed to make, but I still find the idea of a Saturnian follow-up that doesn't focus on Titan rather bizarre.

Going back to Titan is a must, but if NASA plump for a Jovian mission first I won't be too upset as that should also be very exciting. I wish they'd decide, so that ESA can start seeking partners for the other option (including NASA's non-flagship budget).

Post-Voyager score (if you include Juno)

Jupiter. Two orbiters, One probe, Two flypasts
Saturn. One orbiter, One lander.

If you ask me, Saturn's owed an orbiter and some flypasts - another mission to the Jovian system really WOULD be queue jumping.

Like I said - just throwing in a vote for somewhere else. I'd pick Titan over any of the other suggestions. An ideal world, we'd do all of them, but if I had to pick one, just to me, Titan seems the most exciting, interesting and intriguing.

Doug

I stole a little time at lunch to compare camera resolutions. Note: I simply took quoted resolutions at specified distances from public websites and normalized resolutions to 1,000 km distance. View these as approximations.Attached file should preserve table formatting.So with that, here is how the high resolution/narrow angle cameras from various missions would compare at set distances: 1K km 10K km Ganeymede to: Jupiter Io (closest) Io (furthest) Web quoted resolutionJSO 2 m 20 m 2.1 km 1.3 km 3.0km 0.4 m at 200 kmEE 10 m 100 m 10.7 km 6.5 km 14.9 km 1 m at 100 kmCassini 6 m 60 m 6.4 km 3.9 km 8.9 km 0.024 m at 4 kmMRO 1 m 10 m 1.1 km 0.6 km 1.5 km 0.3 m at 300 kmDeep Impact 3 m 30 m 3.1 km 1.9 km 4.3 km 2 m at 700 kmPage 3-5 of the JSO report says that the 0.5 m optics chosen as a baseline could achieve an optical resolution of 775 m from the distance of Ganymede, which is considerably better than is in my chart here. The difference could be caused by (1) a loss of resolution between theoretical optics and what is achieved with an actual CCD implementation (2) I don't know what I'm doing in putting together this chart. Caveat emptor! I stole a little time at lunch to compare camera resolutions. Note: I simply took quoted resolutions at specified distances from public websites and normalized resolutions to 1,000 km distance. View these as approximations.Attached file should preserve table formatting.So with that, here is how the high resolution/narrow angle cameras from various missions would compare at set distances: 1K km 10K km Ganeymede to: Jupiter Io (closest) Io (furthest) Web quoted resolutionJSO 2 m 20 m 2.1 km 1.3 km 3.0km 0.4 m at 200 kmEE 10 m 100 m 10.7 km 6.5 km 14.9 km 1 m at 100 kmCassini 6 m 60 m 6.4 km 3.9 km 8.9 km 0.024 m at 4 kmMRO 1 m 10 m 1.1 km 0.6 km 1.5 km 0.3 m at 300 kmDeep Impact 3 m 30 m 3.1 km 1.9 km 4.3 km 2 m at 700 kmPage 3-5 of the JSO report says that the 0.5 m optics chosen as a baseline could achieve an optical resolution of 775 m from the distance of Ganymede, which is considerably better than is in my chart here. The difference could be caused by (1) a loss of resolution between theoretical optics and what is achieved with an actual CCD implementation (2) I don't know what I'm doing in putting together this chart. Caveat emptor!

For camera resolutions from different missions at different interesting Jovian system distances (1,000s km for moon flybys or distances from Ganymede to Jupiter and Io for long term observations of dynamic bodies) see the attached file. Sorry the previous message got so screwed up, formatting wise.

All other things being equal, I agree. However, in terms of both technology and figuring out a best approach, I would rather put Titan off. I really can't see Enceladus competing with Titan or the Galileans - Cassini seems to be doing such a good job with it.

I might say that not only is Titan more interesting than any of the Galileans individually, it is perhaps more interesting than all of them together, even if you throw in Mars for good measure.

But there's a lot more to do to collect, not to mention analyze, Cassini data on Titan. I'm amazed at how well certain persons (ahem, Ralph) did at prognosticating many of the Cassini-Huygens findings, but this is a world of amazing diversity. We may yet discover a compelling landing site (entailing new instrumentation) that hasn't yet been RADARed.

Europa's data has been placed in the oven and come out a cake. Titan's is still raw dough. Titan definitely looks like a more interesting place, but we shouldn't finish designing a multibillion dollar mission until the second round of papers after Cassini's last gasp. That will be years from now. In that sense, the jovians are once again due.

Very much enjoying the discussion here.

For EE, does planetary quarantine require taking the spacecraft out of Europa orbit at EOM? If so, would it be practical to increase fuel load to move it at least apojove out to lower radiation exposure distance from Jupiter?

Yes, EE would have accumulated its designed total dose by Europa EOM, but this would be about getting some bonus time similar to the last several Galileo orbits. Galileo deliberately went deeper to get the Io flybys. What if EE kept to the outside for some time before making some end of life Io flybys and go out like Galileo? Could EE meet some of the JSO objectives in a post-Europa EOM phase?

Shouldn't that be three flybys? - Ulysses (If you really want to push it, the 2004 distant encounter could bring us to four), New Horizons, and Cassini.