I've been trying to think of creative solutions that could be enabled by ESA possible contribution to an outer planet mission. Here are a couple of thoughts:

ESA has studied a mini-Europa orbiter that would be solar powered and depend on a relay satellite to keep mass and power requirements down. If NASA were to build JSO, then it could orbit one moon and act as a relay for an ESA craft that orbits another moon. Ganymede would be a logical target since the relatively low radiation makes the design easier and it could be solar powered.

NASA has studied a New Frontiers solar powered mission to Saturn that would drop to probes into Saturn during a flyby mission. A possible alternative would be for the craft to carry a Titan probe(s) or balloons supplied by ESA. The craft would have to be beefed up to be an Saturn orbiter so it could act as a relay for the ESA probes. In theory the orbiter could use its communications antenna to continue radar mapping of Titan to help fill in areas missed by Cassini, although the power and communications requirement might move this out of the New Frontiers class. Although the craft in theory could be a near copy of Juno, which fits into the budget, and I've heard that the New Frontiers budget will be increased for the next round.

I personally like the second option. Titan is fascinating.

This discussion is interesting in that it seems to reduce the question down to whether or not Europa alone is intrinsically more interesting than the rest of the Jovian system.

I have to agree with Jason -- I think that, given a choice between the JSO and EE mission profiles, I like the JSO better because it gets most (if not all) of the data you need about Europa to plan the next mission, and gives you a lot more data about the rest of the system than EE would.

Recall, please, that some Galileo results suggest that Ganymede and even Callisto may have "molten ice" (i.e., liquid water) mantles below their solid ice crusts. Granted, Europa is the most interesting from an astrobiology perspective (more access to sunlight and tidal heating), but any body with a hard ice crust and a liquid water mantle can teach us an awful lot about such worlds.

And it may be a very good idea to learn about them -- it's always possible that there are more habitable worlds of the Europa type out there than there are of the Earth type. After all, a Europa could form around pretty much any gas giant in a huge expanse of a solar system's domain, while an Earth has to reside in that narrow little Goldilocks band. Statistically speaking, you might expect more life to arise in Europa-like worlds than on Earthlike worlds, just from sheer numbers.

-the other Doug

The elephant in the room. There seems to be an argument between those
who think the Jovian system has been sufficiently explored in it's whole to
merit the next mission's being narrowly focused, and those who don't.

Well, that's an editorial assessment.

The way I see it, planetary science is a series of games of Twenty Questions and with typical post-Apollo funding regimes, our/likely-anyone's approach has been to pick worlds that are worth playing with and worlds that are only worth a game of One Question. Mercury hasn't been visited in 32 years, so you know which bucket it's in. Mars has five live missions there now, so you know it's in the other bucket.

Europa and Titan are in due time going to get follow-up missions (in the case of Titan, I mean a mission after the mission after Cassini). [I realize this sounds apocalyptic to suggest that some worlds WON'T get any follow-up missions ever. If human existence continues, Eros will probably get a follow-up mission, just in the 26th century or something. I mean on a timeline measured in a couple of decades instead of a few.]

Ganymede would be explored quite well by EE (14 flybys). Compare that to Cassini at Saturn. Enceladus won't get 14 (targeted) flybys by the time the first extended mission is up. EE does not ignore Ganymede. It wouldn't map it into submission the way JSO would, but Ganymede's not in the top tier of interest, either.

Callisto is a wash; seen about equally well by either mission. JSO could provide more long-range monitoring, but what does that mean with a dead world?

Europa, however, is a place where we might want to set a lander down following recon. JSO would be a half-measure in that regard, so we'd be putting it off by decades by flying JSO first. And that's not a winning strategy for Twenty Questions.

I believe a correct paraphrase from Apollo (or the HBO dramatization) regarding finding lunar anorthosite was "It'd be a shame if it were there and we missed it." That's how I perceive Europa, which might have a volcanic Io inside, smoking into a salty ocean, softening the overlying ice shell beyond static equilibrium, opening up some live rifts between blocks of ice you can almost picture a polar bear diving off of.

Or an active fissure where a triple band is widening like the crack on my ex-windshield. Dark stuff spraying up every time the crust flexes.

If it's there, it'd be a shame to put off finding it for 20 years.

Ganymede's going to sit there on ice for another three to six billion years. No hurry.

One or two Io flybys for EE would clinch the advantage totally, but I'd take EE as is. A few views of Io's plumes from 400K km would augment Earth-based long-duration monitoring nicely. Io deserves better, but JSO wouldn't completely nail Io either, so I'd say EE for now and maybe some better Io looks if a combined science/comsat accompanied the Europa lander.

The one good thing about EE is that it would be a lot easier to convince NASA to sign off on an Io New Frontiers mission. 2 flybys is not enough...

(Without reading the voluminous material on the opag site...)

Would a JSO mission have a higher angular resolution camera than Cassini's narrow angle cam?...
Would it be a multi-megapixel camera, 4-megapixel camera on Rosetta, or even higher pixel-count-format?
Would it be able to take images every few seconds, rather than about one per minute?...
Would it have many times more data storage than Cassini, and be able to dump it all to Earth during apoapsis, as it heads in for the next encounter?

As Meriner 10 approached and receeded from Mercury, it "fireshosed" the illuminated disk with continuous mosaicing until the field of view and frame rate became too small and slow to get contiguous coverage.

Voyager was somewhat able to do the same at Jupiter with the flyby-targeted moons.
Galileo was a salvage job.. very successful, but still a salvage job.

A current technology mission should be able to saturate-cover the illuminated disc with multi-spectral images down to maybe 1/4 kilometer with higher camera resolution from long range, a faster frame rate, and more data storage and transmission capability. Maybe even UV/VIS/NIR color coverage at that resolution. During near-encounter, they should be able to do "quadrangle-maps" of target regions with several strips of contiguouis images down to something like 50 meters, much the way Viking Orbiters mapped landing sites with the twin rapid-fire framing cameras and it's scan platform.

I'm not going to vote on Europa vs Jovian System, but we damn near better be able to do as much better at Jupiter than Cassini does at Saturn, as Cassini did over poor Galileo.

Beyond that... Near Infrared imaging is vastly advanced over Cassini's technology level.. A megapixel 1 to 5 <maybe> micrometer framing camera with a good set of wide and narrow band filters should be state-of-the-art and extend multispectal mapping at good resolution to longer, more diagnostic wavelengths.

Yes, Galileo was a salvage job, but it did have some encounters that did not fall into this category, namely its earth-moon flybys, which give one an idea of what SSI was really capable. Also, when one looks at the ultra-high resolution mosaics from its high-speed flybys of the Galileans, the short integration led to images that are neatly nested, especially compared to Cassini mosaics. I would love to see a JSO or EE with a Rosetta-sized (at least) camera with a fast integration time. In fact, it could do more than Mariner 10 if it had reaction wheels, because the coverage could be neatly nested, as opposed to Mariner 10, which appeared to bounce all over the place.

One thing to add. I would quickly defect from my current position if the Europa orbiter had JSO's tour, including Io. This is not OPAG based, but what I would really like to see is a long-term but scaled back JSO style mission, perhaps as small flagship combined with EE. While I definitely consider Ganymede worth an orbiter to the extent I would not oppose such a mission, I like the Ganymede orbit idea mainly because the probe could (especially with an excellent long-range camera) continue to probe the rest of the Jovian system from there. However, if a smaller orbiter filled the long range role, and some Io flybys were added to EE, I would support it. Also, because the technical challenges of a Galileo-II are not as great (in the sense that it is something that has been done before, sans a working antenna), it could be launched sooner and thus could pave the way for EE. However, I realize budgetary reality may not allow that.

I completely agree, and this seems technically feasible, of course. In fact that's the strongest argument yet for merging these mission proposals. Completing a full Jovian system survey should be the primary objective, and after this task is complete the spacecraft should be capable of orbiting Europa or Ganymede, with target selection determined by the survey findings.

Building in adaptability seems like the best way to resolve this conflict. The main idea here is to acquire enough information about the Galileans to make an informed choice when the time comes (though I suspect that the meetings will be real roof-raisers; heck, look how much discussion there's been on this thread alone!)

Even if Galileo's antenna had deployed, there would be a good reason to have the next Jovian flagship mission be able to do good Jovian system science. Galileo's instruments were mid-1970's vintage. Thirty years of instrument development would vastly improve our ability to tackle a number of questions.

Adaptability will inevitably imply sub-optimality for either Europa or Ganymede. You want to optimize your payload (science instruments, rad shielding, fuel) for a specific target, there's no margin like that to accomodate free choice once you get there. It's not a case of choosing the landing site a-la Viking from orbital survey first. You'd inevitably have an impact on science capability to allow for those margins. Once again, it boils down to "do you really want to sacrifice X in order to be able to do Y?". Just as EE sacrifices coverage of the entire system and JSO would leave certain questions open if you went into Europan orbit.

Should that target be a specific body, or a specifc set of scientific goals?

Doug

No question at all that there would be trade-offs, Gordan. However, as vjkane observed, we've come a long way in terms of instrument capabilities. Furthermore, for the satellite orbit phase, we've down-selected to two icy moon targets, so the instrumentation suite need not be radically different for studying either body (only major exception I can think of is that Ganymede needs a magnetometer, and Europa doesn't). Enhanced radiation shielding for a possible Europa mission is still a plus even if Europa isn't the final destination; this could enable Io flybys for one thing, and also generally prolong mission lifetime. Therefore, I still think that designing for versatility is a good bet.

Probably scientific goals. Some goals can be pretty constraining though, suggesting you end up with a target body again. We're back at that point: would we like a little bit of everything (possibly best bang per buck), or do we want a quantum leap in knowledge about a single object?

For this type of mission (in other words, not a mission in a series like the Mars series that launches every two years), we need to make sure we understand how best to approach making that quantum leap. And frankly, if it is a much better understanding of all the Galileans and Jupiter (plus the small moons, since a high resolution long range camera should help there as well) versus a quantum leap at Europa alone with relatively little attention paid to the other moons, I would pick the former. Given all the unknowns, I think a system-wide quantum leap is possible.