http://www.lpi.usra.edu/opag/announcements.html

That's one little URL with a lifetime's worth of reading material.

Three detailed studies are available in PDF format. The missing body is Titan, which will be the subject of a forthcoming report.

The three focus missions are:
Europa Explorer: Fairly detailed description of a mission that is pretty much what Europa Orbiter would have been.

Jupiter System Observer: Basically, Galileo 2 (without the antenna mishap!). The craft would start with a 3-year tour of all the Galileans, then spend 1 year in an elliptical Ganymede orbit, then the rest of the mission in a tight, polar Ganymede orbit (like MGS at Mars). That would map the heck out of Ganymede, but also be close enough to the rest of the system to make long-range observations for years. Note that Ganymede would thereby provide a lot of radiation shielding.

Enceladus: where three profiles are examined in depth: Enceladus Orbiter only; Enceladus Orbiter with soft lander; Saturn orbiter with Enceladus soft lander.

There's more to chew on here than I have had (or may ever have) time for, but I'll throw in my two cents' worth:

Seems like a Europa-only mission would only benefit from coming after a JSO. EE would explore Europa much better than JSO would; why even have JSO observations at Europa if EE came first? In many ways, these two missions are competitive. EE would have the big payoff, but JSO seems like basic recon that would prime EE, especially giving specs on radar performance. But if we waited til JSO was 4 years into its mission before completing design of EE, then put EE sometime mid-century.

If an Enceladus mission included a Saturn orbiter, then maybe the same orbiter could provide data relay for separate Titan elements. However, a lot of the Enceladus science goals would require an Enceladus orbiter, so I don't think a Saturn orbiter for Enceladus/Titan will win out.

Note that Enceladus orbital velocity is low enough that the craft could manage to take lots of hits from ice pellets and survive. Put a bulletproof vest on the craft and let it soar through the plumes endlessly.

Drats. Everytime I try to download the Europa report I only seem to get four pages. I wonder if my Adobe Reader needs to be upgraded. The other reports come down fine.

As I recall the whole idea for a JSO / Ganymede orbiter mission came along a couple years back. The proposal was made to fly a very high resolution imaging package and eventually park it in orbit around Ganymede. The idea was to keep it alive a lot longer by keeping it out of the heavier radiation it would encounter in Europa orbit.

My impression was that it was beleived that it would be cheaper to develope the JSO because it wouldn't need all the radiation hardening you would have on a Europa orbiter. I scanned through the JSO report, and saw budgetary numbers in the 3 billion range.

If NASA headquarters was hoping that these studies would provide them with a cheaper alternative then one of two possibilities comes to mind. Either (a) those hopes were in vain and you just can't save that much by this route, or ( someone didn't get the memo and the JSO engineers decided to go whole hog with a full fledged (and radiation hardened) Flagship study and go for the gold. Gold in this case being 'plated'.

A third more cynical possibility would be that people really want the Europa orbiter and so purposely made the JSO come out just as expensive and therefore no more appealing from a budget standpoint.

Please note that I'm NOT advocating that third option... just saving someone else from suggesting it. I'm leaning more towards option (a) as an explanation.

I think the Europa report is only four pages. That's all I see.

JSO would certainly average lower radiation per day than EE (dose at Ganymede about 5% the dose at Europa), the plan is for a very much longer mission, so it will take a high lifetime dose. A short JSO mission would take very little radiation dose, but also have not much point.

JSO sounds like a good mission to me, on the Galileo/Cassini level. But its competition with EE puts it in strange turf. I don't see it taking root.

It's not your Acrobat reader -- I get just four pages, too.

As for JSO, I spent some time going through the budget figures. There's really two missions described. One uses up all the potential flagship budget (~$3B) and the other reduces the amount of Ganymede science through some instrument descope and an elliptical rather than circular orbit. The descoped mission appears to be about 2/3 the cost of the proposed mission (there are more expensive versions, for example with an atmospheric probe). See figure 3-4 on page 3-5. The report also says that no attempt was made to define a minimal acceptable mission, although some descope options are presented that suggest ideas.

My take is that once you get to this class of mission, you are in $2-3B range, and the choice of moons doesn't matter that much.

It is a shame the Europa report isn't up. I'd like to compare the science packages. From the JSO report, it appears that two instruments weighing about 100 kg are needed to enable long distance observations (a combined camera/NIR spectrometer and and an IR spectrometer). I think it would be criminal to return to Jupiter with a 3-axis stabilized craft and not include these instruments plus along with flybys of Io and a long orbital tour to observe satellites not to be orbited plus Jupiter.

Some more details comparing the Europa explorer and JSO options.

There's a very good detailed description of Europa Explorer's strawman instruments and the way they would be used -- as of last May -- at http://www.lpi.usra.edu/opag/may_07_meetin...gship_study.pdf . You'll notice that they are very similar to those in the JSO Flagship final report, with the following differences:

(1) The final version of JSO calls for a half-meter aperture telescope with the Narrow-Angle Camera and Vis-NIR Spectrometer hooked up to it, for a resolution of 0.4 meters/pixel from 200 km range for the Camera. (This is actully a change from the design of JSO's strawman payload last May, which called for separate optics for the camer and spectrometer -- the result being that they both had considerably lower resolution while weighing only 5 kg less total than the new setup.) Europa Explorer as of last may called, by contrast, for a Narrow-Angle Camera with a resolution 5 times poorer, but weighing only 15 kg -- plus a separate Vis-NIR spectrometer weighing 30 kg. I rather expect to see a similar combined-optics system now adopted for Europa Explorer (whether it features a comparably big telescope or not).

(2) JSO contains a big hulking Thermal-IR Spectrometer weighing fully 43 kg -- whose separate IR optics, in fact, are as huge (0.5 meters) as those for JSO's NAC-VNIS combination! (This is also a new change; last May, the TIR Spectrometer was to weigh only 20 kg but was to be accompanied by a separate 15-kg Thermal Imager.) Clearly good high-res thermal-IR spectra are considered very important on this mission, in order to study Jupiter's atmosphere and Io. Europa Explorer, as of my latest info, carried only an 8-kg Thermal Imager. So presumably a full-fledged Thermal-IR spectrometer is the most likely instrument to be listed for JSO but rejected from Europa Explorer -- since the latter is supposed to focus more on Europa, with general Jupiter-system science being a lower priority than it is for JSO.

(3) The Medium-Res Stereo Camera on JSO, again, has a resolution 5 times better than the stereo camera on Europa Explorer (plus a pixel swath twice as wide) -- but weighs twice as much. (Europa Explorer would also carry a 3-kg Wide-Angle Camera.)

(4) The Plasma and Energetic Particle Spectrometer on JSO weighs twice as much as the 12-kg one on the Europa Explorer (although it uses only a little more power) -- but Europa Explorer would also carry a separate 15-kg Ion & Neutral Mass Spectrometer, which is not on JSO, to analyze substance sputtered off Europa's surface by Jupiter's radiation. (This is the lowest-priority instrument on Europa Explorer.) I don't know what to make of this difference -- since the Europa Explorer plasma instrument, unless it's descoped, would also have ability to make time-of-flight analyses of the composition of plasma. Presumably the JSO version is more sophisticated in its sensitivity or resolution.

So, really, the instrument payloads for the two missions are strikingly similar. The main differences seem to be higher optical resolution for the cameras and near-IR spectrometer on JSO, and the absence of a thermal-IR spectrometer on Europa Explorer. All of which is to some extent changeable; the final version of Euripa explorer might easily end up carrying a thermal-IR spectrometer and combined optics for its cameras and spectrometers, although I imagine these would all be lighter-weight and lower-capability than their equivalents on JSO.

One more important note: it was stated at the Icy Moons Workshop that JSO would carry just as much radiation shielding as Europa Explorer, in order both to allow those Io flybys (reduced from four to three in JSO's latest version) and to prolong its lifetime in the Jupiter system for as long as possible. The overall features of the final version of the JSO spacecraft and that of May's version of Europa Explorer are very similar:
Europa Explorer: 7225 kg total mass, 2608 m/sec delta-V
JSO: 7262 kg; 2705 m/sec

The main differences are that JSO would carry a lot more total instrument mass (310 versus 212 kg), an 8th MMRTG to power the craft, and of course more area of radiation shielding to cover the extra instruments (although, since JSO would only be designed to endure 1.8 Mrad versus Europa Explorer's 2.3 Mrad, the shielding would be thinner and so its total mass would be virtually identical -- 165 versus 162 kg). The buses for the May version of Europa Explorer and that of the current JSO -- minus their science instruments and radiation shielding -- have almost the same mass: 1889 kg versus 1934. But the new JSO has not only more science instrument mass (310 vs. 212 kg), but considerably more radiation shielding (243 kg vs. 165) -- so that its total dry mass is 220 kg higher than that of Europa Explorer.

Also, JSO carries more propellant for the additional 97 meters/sec of delta-V it required for its mission. All this is apparently possible because the 2017 launch opportunity listed for JSO is a good deal better than the 2015 window listed for Europa Explorer: 7810 kg capability versus 7225 (using a Delta IV-heavy and a VEEGA trajectory in both cases). So: delay Europa Explorer by a couple of years, and you could put a lot more on it, too.

One other difference: JSO's final version allows mission data rates of fully 600-1600 kbps, whereas the May version of Europa Explorer provided only 200-300 kbps -- but this seems due to the fact that JSO would have a 50-watt Ka-band transmitter with a low-powered X-band backup, whereas the May version of Explorer used a 50-watt X-band transmitter and only a 3.5-watt Ka-band one. The power requirement of JSO's version is only modestly higher, and its mass is actually slightly less than for Explorer's version -- so I imagine the final version of Explorer will be changed to the JSO setup.

We seem to be looking at only minor differences between Europa Explorer and JSO: the latter would trade off somewhat thinner radiation shielding to allow more instruments an 8th MMRTG, and maybe a somewhat higher com rate. And, even then, the only significant instrument differences are (as mentioned above) just smaller optics for the cameras and VNIR spectrometer (which Europa Explorer would partially compensate for by lowering itself into a 100-kg attitude Europa orbit, versus JSO's 200-km Ganymede orbit), and the lack of a full-fledged thermal-IR spectrometer.

Again this points out we live in exciting times, with upcoming missions near Mercury (Messenger 2011), cometary & asteroid flyby (Rosetta 2011 + 2014) and the flyby of the best-known KBO Pluto (New Horizons 2015). Exciting times indeed!

Thanks for breaking it down.

Another aspect to the better optics for JSO is that the studies of Io and Jupiter from long range will benefit directly from the improvement in spatial resolution. Ganymede itself doesn't warrant such a commitment in hardware, and it basically "lucks out", becoming one of the best-mapped bodies in the solar system because this mission architecture doesn't really sacrifice anything by providing that capability.

Added note on the JSO report: Its discussion of radiation exposure assumes no shielding from Ganymede, even though it is expected that close proximity to Ganymede will provide net shielding. A priori, I would have guessed that it would block almost 50% of the charged particle flux. I know that Ganymede's own magnetic field makes that somewhat unpredictable, but I still figure that about half the time, Ganymede would stand in between JSO and the "radiant" of charged particle flux from Jupiter's field.

I guess the way this will end up shaking out is that EE will fly soon-ish, and JSO will fly way later. It would be nice to tailor their durations so that more or less continuous coverage of Io and Jupiter's temporally-varying phenomena could be provided, or even to have both of them operating initially simultaneously to provide excellent studies of the particles and fields sorts of things, but that would entail a lot of billions being spent at Jupiter while Saturn's retinue waits for a follow-up.

So I think EE will be the next Outer SS flagship mission and the following slot will probably go to Titan. JSO may be able to outshine Enceladus for the next spot, and then perhaps tailor its tour to provide more unique closeups of Io and Callisto while ignoring Europa and Ganymede which will be otherwise covered thoroughly.

Other Outer SS priorities worth considering include Neptune orbiter and yet other Europa/Titan follow-ons. It's not a question of whether or not something interesting will be ignored, but how many interesting things will be ignored (for the next 40-60 years or so).

The full Europa Explorer report is now up. More reading (282 pp) material for your bedside stand.

Baseline mission is a jovian system tour of about 2 years followed by a 92-day primary mission in Europa orbit, with high expectations that the craft would survive considerably longer. Presumably, other targets in the jovian system would get minimal attention once Europa orbit was achieved, since more Europa coverage will always be desirable. (Eg, stereo/laser topographical map will get denser and denser with more observations.)

14 Ganymede flybys and 4 Callisto flybys before Europa orbit. Ganymede is highly useful for gravity assists, but the scientific value of Ganymede flybys would be severely lessened if a JSO mission were ever to fly.

JSO thus becomes a mission where Io is a major consideration, although Ganymede would be better observed. Deep in the radiation belts, Io doesn't fare well as a flyby target according to the real estate rule of Location, location, location.

Sweet! Obviously, the two that I favor are JSO and the Titan Explorer. Of the two, I am pulling for JSO, but I wish both could be selected. JSO seems to be the best mission WRT science/dollar. Yes, it seems a lot like Galileo 2, but I don't think that should be considered a bad thing. We don't stop sending orbiters or landers to Mars, for example, simply because it was done before. With each mission, we understand what questions to ask and what instruments best answer our new questions. JSO will end up doing quite a bit of Europa science that will answer the most pressing question about that world (that an orbiter can answer): what is the thickness of the ice shell? My only problem with this mission is that it could make it more difficult to justify a dedicated Io New Frontiers mission, but barring an opening up of the mission selection process for NF, JSO maybe the Iophiles best hope for the next couple of decades.

The Titan Explorer is probably a close second for me, mostly because it is a mission dedicated to only one object, so I think the science/dollar is a bit lower, but considering the diverse geology and atmospheric science that could be obtained from this mission, it would still be a spectacular mission. Still want to see what the baseline mission looks like. An arrival date in the mid-2020s would be ideal to hit around equinox.

The Europa Explorer and the Enceladus mission, I think, are tied for third. I think a choice between those two missions would have to wait till the end of the Cassini mission. By then, hopefully the question of the source of the plume will be resolved. If the source is in fact a liquid water reservoir, as Porco et al. suggest, I think this mission just way ahead of Europa, given that samples from such a reservoir could be much more easily obtained, in a single mission, than Europa. If the source is much more mundane, as Kieffer et al. suggest, then a mission to Enceladus becomes a much lower priority. For Europa, while the orbiter mission is quite interesting, its focus on Europa pre-EOI and the fact that it will not answer the most pressing question about Europa (the presence of life), I think makes its science/dollar much lower than JSO.

would you vote the same way if the JSO long distance remote sensing instruments (big camera/NIR spectrometer and IR spectrometer) were on the Europa orbiter? Assume that the two missions (JSO and Europa Explorer) were combined, both did Io flybys, both flew by which ever moon wasn't orbited a lot of times, and the only difference was which moon was orbited at the end of the mission.

Yes, if the only difference were which moon was orbited, then yes, I would change my vote to the Europa Explorer. However, keep in mind that another thing that makes JSO interesting is that it can orbit Ganymede for a much longer period than EE could due to the greater radiation exposure at Europa. This provides a much longer opportunity to observe Io and Jupiter which maybe more important than the resolution gained by being at Europa (plus, with a much shorter mission at Europa, more than likely there were would be few if any observations of the rest of the system while in Europa orbit).

I've taken a look through the JSO and Europa Explorer proposals. My take on the choice for the next Flagship mission is that there are two classes of missions being proposed. The first class is to a single moon: Europa, Titan, or Enceladus. (While the Europa Explorer will do some Jovian system science, it's instruments are not optimized for that purpose.) The JSO mission would be a true Jovian system mission with a Ganymede mission in addition. It appears to this arm chair engineer that JSO could be flown to either Europa or Ganymede with fairly few changes.

In my opinion, if the next mission is a focused moon mission, Titan is the most interesting of these three options, and the proposed Titan mission with an orbiter, lander(s), and possibly a balloon (I read somewhere that this may be dropped from the proposal, but am not sure) seems very capable. However, if given a choice between a Jovian system mission that ends orbiting either Europa or Ganymede, I think this is the most science for the buck.

As described, here are the difference in outcomes:

Jupiter: Long-range reconnaisance more frequently and for a much longer time with JSO.
Io: Long-range reconnaisance more frequently and for a much longer time with JSO. About four well-distributed close flybys with JSO. No close flybys with EE (several nontargeted flybys about 400K km in distance all with similar geometry).
Europa: "Total" mapping with EE. About 7 close flybys with JSO.
Ganymede: "Total" mapping with JSO. About 14 close flybys with EE.
Callisto: Roughly the same outcome in either mission. (Slightly better with JSO.)

Here's my question: How much of the Io long-term observations CAN'T be done from Earth for a ton less money? AFAIK, Io sports about ten major eruptions at any given time, and those are observable from Earth with adaptive optics. For a fraction the cost of JSO, build about 5 tropical observatories dedicated to Io, such that two or three of them can view Io at any time. And, hey, observe Jupiter, Uranus, Neptune, etc., every few hours as well.

If you run a program like that, then JSO looks like a distant poor stepchild to EE in science value. Sure, it would provide some nice close-ups of Io, but Galileo didn't totally skimp on those. And the value of those would be more than made up for by the fact that EE would provide "total" mapping of a world of primary interest (Europa) instead of a world of secondary interest (Ganymede).

The key with Europa is that a survey with selected, partial coverage at top resolution might miss a landing site of unique value, eg, over a hot spot where the ice shell is thinner/softer. None of the other moons have any potential payoff of that kind, and JSO would only provide a chance at finding such a place if it exists.

I'd say that EE is the better mission by far if we do what we should and track Io (and all the gas giants) consistently from here a few AU away. I'd add that I see a lot more value to a few Io flybys than the 14 Ganymede flybys if the EE plan could be tweaked like that. Plan JSO for the distant future, and EE could plan on skipping all Ganymede science except where programmatically convenient (gravity assists).

My lone-wolf conjecture on Europa vs. Enceladus is that Europa's ocean is likely more interesting than Enceladus's because it's "dirty". Even if water is key to life, we can also say that 100% pure water is inherently incompatible with life. From what we've seen so far, Enceladus's water is not quite 100% pure, but doesn't feature many complex compounds. I could see it being a reservoir of window-washing fluid of no astrobiological potential. Meanwhile, we can *see* that Europa's innards have some interesting goop in them; otherwise, the triple bands would be white, not tan.

Titan deserves a heck of a mission, too, but I think it'll merit waiting for Cassini finishing its reconnaisance before completing that design. Europa had its last close-up about a decade ago; it's due.

Interesting question. I think there is a lot of great science to be had from planetary monitoring
(20 mins per target per night would do fine in most cases) of io's eruptions and clouds on
Titan, Uranus and Neptune, plus Venus nightside, and Jupiter/Saturn by an observatory system
like you suggest . But to do a good job these need to be 10m-class facilities with AO...not cheap
and NASA is in the business of building spacecraft rather than infrastructure.

A lot of that science could be had by more creative scheduling of existing facilities (e.g. allowing 1/20
of a night allocations for months on end..)

(A retort question - how much more science would you get by beefing up (or even restoring to
higher reliability) the DSN - allowing you to downlink more (and/or lose less) data from existing missions.
I bet in terms of science/$ it is a good expenditure, but infrastructure is never a sexy item to
sell)

As for Io - you can see eruptions are there from the Earth, identifying the location, measuring the total
heat flux in a number of bands (allowing area/temperature estimates) but that doesnt give you any
of the geomorphology, plume dynamics etc that a JSO would give you.

I tend to favor JSO due to the better Io coverage, not to mention the Ganymede coverage. However, while I find Europa interesting, I have never been on the Europa bandwagon. To put it another way, I find it one member of the Callisto-Ganymede-Europa-Io series, and to understand these worlds, obsessively focusing on just one of them (at this point, at least) seems a poor choice.

I tend to agree. Whereas EE is really a fixed-term suicide mission due to the Europan radiation environment, JSO could potentially survive into a very long extended mission in Ganymede orbit, continuing to monitor changes in every part of the Jovian system - conceivably including active processes on Europa. Another thing: I'm significantly more uneasy about dropping litter on Europa than on Ganymede.

On the other hand EE is at a more advanced stage of planning and is also an enormously exciting proposal. I'd hate to have the responsibility of deciding.

Although I'm not really a Europaphile myself, I'd have to vote for EE as the pick of this litter. Not only would we get nice new coverage for the other Galilean moons (and remote monitoring of Io), but remember that Europa's been declared a high-priority objective in close alignment with one of NASA's top-level science goals. Therefore, EE would probably be easier to sell to senior management then the other missions.

(My emphasis here is on getting an outer-planet mission in the pipeline ASAP; we're looking at quite a gap already after Cassini & NH).

Nigel, you're right; I don't envy the task of whomever has to choose amongst these proposals, I want to fly them all. Is this Alan's new job?

Depends. Both would do Europa science. However, it is my understanding that JSO is significantly cheaper because of not having to stay as far inside the Jovian magnetosphere for an extended period of time. It also would, for the same reasons, not require as much new technology. And frankly, I think it would help us better select instruments and priorities for future missions. Galileo's coverage was so spotty that you can't convince me that there aren't new major areas of interest we are missing. I mean, look at all we have found on Mars with imagers since the Mariners and Vikings? Galileo and Voyager coverage of the Galileans doesn't even begin to compare. For instance, JSO might discover plumes eminating from Europa a la Enceladus (which would be much smaller because of Europa's greater gravitational pull). If so, it would be desireable to have appropriate instruments for in situ studies (sort of a below-the-ice freebie, although it wouldn't exactly be pristine material after being propelled into space). Galileo did a plume search, but it was never likely to suceed, given the extremely limited number of images it was able to take. JSO could even continue such a search from Ganymede if it was deemed desireable. My point is that Europa could still be used as a selling point.

True, identifying the funding party is an outstanding concern; some of that work is taking place now, from the ground as well as HST; basically, NASA won't make this solution happen, but it may end up happening anyway, which would have the same effect on shifting the comparative values of various spacecraft options.

True also that JSO would provide far greater resolution than an Earth-based campaign, but some of that would be compensated for with EE observations of Io. No doubt, JSO does a better job than Earth-based + EE; I am just trying to whittle down the difference to the point where it doesn't outweigh EE's far better reconnaisance of Europa. (Minus JSO's far better, bordering on excessive, reconnaisance of mid-level priority Ganymede.)



Even a few losses in data return, amortized over mission costs, probably would pay for a lot more in the way of DSN sites. That ratio will probably increase as faster onboard electronics with greater storage capacity lead to outer-planet missions that have enough data to spend the whole apoapsis portion of the orbit transmitting data without running out of things to say. (Eg, a Kayuga-style HDTV system on a Galileo/Cassini kind of mission.)

What would make infrastructure sexy would be if it made for good pork barrel spending. Stations in Spain and Australia can't do that, but stations in Hawaii and North Carolina could.

Hmm. That's a very persuasive point, Ted; we haven't really completed a Cassini-quality survey of the Jovian system yet. Might change my mind here, gotta think about it.

Two things that I would really like to see are long term monitoring of Jupiter by remote sensing instruments, with high resolution movies of clouds and lightning (on both the day and night side), which was the science goal most damaged by Galileo's antenna problems, as well as high resolution multispectral mapping of the moons. Galileo's color coverage was awful (except for Io, but this was low resolution). Most color images that were returned are low resolution color images overlayed on a high resolution image, which can be deceiving. Often the images were made by taking a full pixel resolution green image (often with lossy compression) and then taking the other colors with more compression plus 2x2 binning. The 2x2-binned color data would be overlayed on the green image. Not only does that make the color boundaries seem less distinct, but it means that the grayscale is based on only one filter, which wrecks havoc on colorful worlds. Ganymede is full of tectonic features, but also appears to have many largescale color variations. It may prove quite interesting to map these with more filters and at high resolution. We may discover something completely unexpected.

Building on this, I fear that a mission like JSO may never fly if EE flies first. And again, I disagree with the Europa fettish (at least at this point). The sequence reminds me of Rhea-Tethys-Dione-Enceladus (and several others, such as Mimas and Iapetus may or may not fit in there somewhere). They are worlds that formed out of the same stuff that have had very different amounts of activity. Trying to understand one of the moons with only spotty coverage of the others would be a mistake (Granted, for the Enceladus mission, this wouldn't be a factor, since Cassini is providing excellent coverage).

True. However, even if there's some important area of interest waiting for us there, there's one that was clearly already identified - Europa. Should we dismiss it some more just so we can go chasing other (possibly) interesting stuff in the Jovian system or concentrate on scrutinizing Europa right now? I'm in favor of the second option. The other areas will pop up eventually anyway. Studying Europa potentially has much bigger implications biologically and even philosophically than, say, mapping locations of Ionian volcanoes (I probably alienated quite a few folks now).

JSO (from what I understand) would provide us with a very rounded-up investigation of the Jovian system, but it probably wouldn't amount to a "quantum leap" in knowledge about any particular object. Winding up in Ganymede's orbit, it would give us awesome coverage of the moon. That's great, but is Ganymede really that worthy of a target? Compare this to EE which wouldn't give as a nice round-up like that, but it sure would provide for that "quantum leap" and for an object that is identified as a high priority, likely solving questions that were raised since Voyagers flew through the system.

I get a feeling (maybe unrightfully so) that JSO would be a sort of Galileo on steroids. Voyager flybys gave us that "wow, the Jovian system is very interesting, we should go into orbit there". Then came Galileo and we basically went "wow, Europa is really intriguing, we should go into orbit there". That's why EE, not JSO seems like the next logical step to me.

I'm a sucker for cool imagery as much as the next guy and would like to see great coverage of all the moons out there, but there's more to it than mapping surfaces. As much as I like images of colorful Io, Ganymede and Callisto, I want to find out what's beneath that frigid ice crust of Europa even more. If there is anything except more ice. Even if that means having to look at more greyish-bland imagery than at other Galileans. I wouldn't call it a fetish or some trendy thing to want to understand Europa, the moon's under the microscope for a good reason IMO.

There oughta be a metric of diversity. Deimos seems to have a lot fewer terrain types than Earth. Some worlds, you haven't seen it all til you've seen it all. Others, any sampling of the surface will do.

In terms of the Galileans, Callisto is most likely to be homogenous. The difference between a dedicated orbiter and two good flybys might be how much of the same stuff you see. Ganymede is much more interesting, but it would be hard to prove that the same isn't true there: maybe one close up of the grooves plus km-scale global mapping shows you the full set of variety.

Io, in any of these plans, gets a compromise approach: You can't observe Io up close for a very long time, even though you'd like to. So you have to observe it up close in glimpses, then for a long time from afar. JSO obviously does this better than EE; I think reading between the lines, you see JSO as mainly an Io mission that happens to spend all of its time in bed with Io's sister.

JSO would also be very useful for good plume search surveys of Europa, but that's a particular kind of investigation that may be a wild goose chase. Europa is one of the very few outer SS worlds that will eventually merit a multi-mission sequence, so we want to play Twenty Questions very carefully when the questions cost a couple of billion dollars.

Ultimately, I think Europa's likely diversity is what makes it the key target of interest. I keep thinking about (but not mentioning) Conamara Chaos. This is where we saw the sliding ice blocks that said so much about the crust and its dynamics. Conamara is tiny -- only 100 km or so. There are other areas of Europa we still never have seen with decent resolution, and they may contain a half-dozen more Conamaras -- or sites much more (recently) active than that. With this in mind, I want to see us absolutely nail Europa before we spend a billion dollars anywhere else in the jovian system. JSO would improve our Europa coverage, but it would mean we'd STILL need to launch another EE-style mission before the next Europa mission.

Given that, I'd argue for EE now and then after that contemplating a mission architecture where a JSO-like mission could serve as the comsat for a Europa lander.

I'm not normally part of the astrobiology mafia, but Europa has a surface about as old as Philadelphia whereas the other icy Galileans have surfaces as old as the Moon. Io obviously is a seriously competing interest (fraught, unfortunately, with perils), but Ganymede and Callisto aren't even in the same league as Europa in terms of follow-on interest.

I'd consider adding Io flyby(s) to EE -- later launch dates could improve mass margins so that more shielding could be added. But if EE doesn't fly next, then we're postponing an endgame which is potentially the most interesting in the solar system -- there could be dead bacteria in that dirty ice, waiting for a microscope to see!

But what makes it so much better than Io or Ganymede? So much more interesting that it seems like a logical next step? Trust me, it isn't those cool fractures... It is the ocean and the possibility of life. JSO has a long-wavelength radio antenna so it can sample the thickness of the ice. Sure it can't map variations like EE could, but it would be enough to answer the question of how thick that ice crust is. As far as the life question goes, let's say it all together now: Europa Explorer IS NOT(!!!) an Astrobiology mission. It will not find life. It will not search for organics. Those will require a follow-up mission: a lander (now that is your logical next step, and one that makes sense). Err, I am starting to sound like He who shalt not be named...


What do you think you will get from EE for the most part: a lot of images of fractures (not saying that is necessarily a bad thing, but let's call a duck a duck).

I am not saying that EE is bad mission, just that the amount of science it will accomplish per dollar spent is less than JSO, IMHO. Most of the important questions at Europa that can be answered from orbit will likely be answerable with JSO as currently baselined. All the "cool" questions will require a lander.

I never once said it's going to be a search for life. I don't expect it to go diving into the ocean. It WILL be able to characterize the ocean (if any) much, much better than a flyby mission would. What makes you think an ocean would have a uniform crust depth everywhere? Several radio measurements with flybys would give you samples, not a global distribution. EE would be able to perform global high-res imaging JRehling talked about (Conamara Chaos etc.) - identifying potentially good candidate sites for future landers and it would provide much better high-res imaging of the actual surface topography at those sites. I'm aware the imagery would be basically cracks and more cracks - as I said, I'm ready to live with that.
As for a lander being the next logical step, you really wouldn't want to send anything down before mapping the surface topography pretty well, and Europa obviously is very rough at small scales. Once again, something that wouldn't be done adequately via flybys - too sparse a coverage.

Hmm again...keeping up with all these great arguments is challenging, but most enjoyable!

Maybe a compromise is feasible. It really seems as if we're talking about two missions with largely similar capabilities in terms of payload. The question then becomes "which moon should *** orbit during the terminal phase of the nominal mission?" I think a merger of requirements is quite possible, and I have to side with Gordan on Europa being the most desirable moon to orbit given our present state of knowledge about the Jovian system.

Of course, this may change based on the findings of the mission itself. IIRC, Ganymede has an OH- torus, which might indicate outgassing. The key would be to design a mission that has the option of orbiting either moon, and therefore building in the radiation hardening regardless.

I think that the discussion of which moon is more interesting combines personal interest with scientific importance. For me, Io is *personally* the most interesting of the moons. Each of us probably have our favorite moon based on some emotional reaction.

As for which moon is scientifically most interesting for mission that will orbit one, here is how I look at this:

Io would tell us a lot about early stages of being a silicate planet, but you can never orbit it because of the radiation

Callisto is too homogeneous and doesn't exhibit a wide range of icy moon/planet forming stages

Ganymede is heterogeneous and exhibits a wide range of formation stages. Studying it reveals a lot about large icy moons (including, I suspect, Titan and Triton)

Europa is a special case without other analogues in the solar system. If we plan to explore it on/under the surface in the lifetime of my 18 year old son, then it should be the target of the next orbital mission.

Whatever moon is chosen to orbit, I think the craft needs capable instruments for long range studies of Jupiter, Io, and better coverage of other moons from flybys. The current EE instruments seem lightweight for this, IMO.

I still fail to see what makes Europa so much more interesting. An outside chance at microbes that EE won't detect but might lead us to is interesting, but from my perspective, understanding the overall evolution of the Jovian system and the solar system as a whole is more interesting. Also, with a high transmission rate, combined with a camera a lot of pixels and a fast integration time, could provide excellent coverage - Galileo gives us a distorted perspective. Also, I support JSO for the same reason I like DAWN. I knew we would pick a Vesta mission one of these days, but I wasn't sure if we would go to Ceres in my lifetime. I am sure we will go to Europa sooner or later, but I am not so sure about a mission to Ganymede unless JSO flies.

The thing is, Europa has captured not only public imagination via popularization but also the interest of very influential groups such as the AAAS; that's a pretty tough current to swim against with respect to pitching mission proposals. (We gotta talk realpolitik, unfortunately, when discussing issues like this; I don't like it either, but it is what it is).

However, gonna restate what I said earlier: A good compromise would be an orbiter that could be targeted at the end of an extensive Galilean tour to orbit either Europa or Ganymede, with target selection largely based on mission findings up to the decision point. I completely and utterly agree with you that it is entirely possible that many more surprises await discovery in the Galileans, and therefore that a Europa bias may be premature; allowing some versatility in EOM scenario selection is the best option to maximize science return. (Unusual result in current times: the EOM phase is likely to be the most interesting of the entire mission!)

I am not sure the public even knows what Europa is. I really think it would be a shame if astrobiology and hype guides mission selection. The thing is, Europa's radiation shielding requirements are much greater. I wonder how much a "Galileo-II" style mission that examines the moons and then finishes off with trying to fly through the inner Jovian system as many times as possible like the original Galileo did would cost.

Wow, this debate has really taken off!

Without denigrating EE I would just like to address the question above. My answer is yes. If we make the pessimistic assumtion that the Europan ocean is not full of jellyfish then a geophysical study of Ganymede and the dynamical history of the whole Jovian system become the main research targets. Topography old AND new, the whole history is there. Magnetic dynamo - how does it work and what protection does it provide? Long term vantage point: perfect.

Ted, when I said "public", should've said "aware public"; they're out there, mostly Discovery Channel watchers here in the US....and they vote, and they write letters, and they're usually affluent. Not a large consituency, but often quite vocal.

Fact of the matter is that there have been numerous TV programs about Europa, so the buzz is there. Question really boils down to how to design a mission that maximizes science return while simultaneously garnering enough funding & support to actually materialize? Not an easy problem to solve.

With all due respect, should we really be making such assumptions? With foreknowledge, 20/20 hindsight and whatever, then yes, a Ganymede orbiter would make more sense. We just don't know what's out there, that's why we want to send missions there in the first place.

Isn't that kind of like saying "if we assume Titan and its surface is not that worthy of closer scrutiny (i.e. dull and dead, despite evidence to contrary), we can concentrate on investigating other wonderful moons in the Saturnian system"?

Man, this thread is on fire...great stuff, though.

Gordan made a great point about assumptions. It's arguably true that right now, after Cassini's efforts to date, we know more about the Saturnian system than we do about the Jovian. Given the fact that the state of the art has advanced, it makes sense to do a similarly-scaled survey of Jupiter & its environs, with the icing on the cake being a detailed survey of either Europa or Ganymede at the end of the mission.

This is the versatility that I and others have wished for Cassini, but it can't happen. However, a blending of objectives & concepts for JSO & EE might achieve this.

Again, the problem is that the EE has nothing to with any possible jellyfish in Europa's ocean. EE is caught in a catch-22 where the whole push to go to Europa is to look for life, but EE won't have anything to do with that question, but we may need the orbiter to look for good places to land to answer that question. I think it might be useful, as vjkane has suggested, merge JSO and EE to get the best of both worlds: get the added science that the pre-EOI (or pre-GOI) orbital mission would provide at Io, Jupiter, and the other moons, then get the orbital mission at Europa. This would add to the cost of EE, but it might be worth it.

The only problem right now with the Europa "vision" at NASA are the timescales involved. The Mars people are complaining about a possible gap in Mars mission in the next decade to pay for MSR; at Europa, we would have EE in the 2017-2027 time frame, and the lander in the decade after that...

That's assuming the orbiter would find anything interesting enough to merit a landing... I see your point about funding and prioritizing, though.

We'd all like to have JIMO back so everyone'd be happy, but this is not an ideal world. To be frank I'll be happy to see the day any flagship Jupiter-bound mission gets off the ground. If it turns out to be focussing on Europa, all the better.

Just to add more fun to this debate, a long time ago when a Europa orbiter was a new idea I read an AIAA paper on possible implementation architectures. The majority of the paper talked about implementing an orbiter. However, as a backup option, an arbitrarily large number (I think this paper discussed a dozen or two) flybys of Europa could be done. Using gravity assist, the encounter point with Europa in its orbit could be changed. In this way, you can have the closest flyby occur at any number of longitudes at Europa. Apparently it was harder to vary latitudes.

Fast forward to the $1B Enceladus flyby study. It had a number of flybys of that moon that walked around latitudes and then modified the Saturn orbit to approach Enceladus from the other side. This way, both hemispheres would be observed.

Now look ahead to a Jovian orbiter. It's mission could be tweaked to make a large number of flybys of one moon and then orbit another. The primary costs would be mission operations, fuel, and radiation exposure (especially if the moon with lots of flybys is Europa).

So get on your wishing caps. You almost can have two intensive moon studies (plus remote studies of Jupiter and Io and maybe some Io flybys). Imagine what the JSO instrument set could do with 24 flybys of Ganymede or Europa spaced 15 degrees around the equator. If the flybys are close enough, you can get snippets of coverage with the radar system.

Here's my rundown (pretty much mentioned by vjkane - I apologize if I repeat/paraphrase too much):

Pros (what we think we'd learn going into the program)/ Cons (mission lifetime issues)

Io - volcanic and tidal flexing of silicate bodies/ high radiation environment
Europa - relatively boring ice geology, potential for future astrobiology missions / radiation environment
Ganymede - big planet diverse geology, will teach us about geology on large ice moons/extrasolar planets / easier radiation environment
Callisto - relatively boring geology / easiest radiation environment

The lessons from Ganymede formation and geology can be extended to large differentiated ice planets such as Titan, Triton, (and Pluto - which will only get a flyby), as well as new icy planets in extrasolar systems. IMHO, a Jupiter explorer that ends up with a majority of coverage at Ganymede will pluck the scientific low-hanging fruit and get us the most bang for our buck. That should be the first mission back to the Jovian system. We will develop knowledge about a whole class of planets.

Europa is astrobiologically interesting. We need to go there. But there is lots of technical work that needs to be put in place to survive a lengthy stay in that environment and thoroughly examine possible landing sites from orbit. This would be a good next step in the Jovian system.

Follow-on Jovian missions should then target landing and drilling into Europa. These missions would be very complicated and the technologies to do this would need to be developed to assure mission success.

The first science we can easily do from orbit is geology, not astrobiology:

Ganymede will teach lessons about the geology of whole class of large ice worlds.

Europa will teach geoloby lessons only applicable to Europa and Enceladas, and maybe to tidally preturbed small moons in other systems (which we won't be able to detect for a very long time).

I vote Ganymede (JSO), then Europa for the next Jovian mission.

-Mike

I'm surprised that almost the entire debate seems based on the idea that the only thing interesting about Europa is that there might be life below the ice.

Have any of you looked at the pictures? Or read the articles? There is lot going on down on Europa. Cycloidal cracks. Chaos terrain. Possible periodic geysers spewing up through the cracks, staining the surrounding terrain with various salts and compounds.

Even if you throw the whole "life" thing out, here we have a planet sized moon, a huge ice pack covering a global subsurface ocean, with a large diversity of geologic activity. And pretty darn unique. One of the things that makes it so fascinating is that no other object in the solar system looks anything at all like it.

Now, as to what can you learn with orbital vs. flyby?

There are at least two things you can't do with flybys. A laser altimiter in orbit can measure the flexing of the surface due to tidal forces as Europa moves through it's orbit. That flexure is the most reliable way to determine how thick the ice shell is, the radar system is really unlikely to penetrate the shell if it is more than a couple kilometers thick.

The second item is a magnetometer which can measure the induced magnetic field in the European ocean. In all of Galileo's flybys they only detected a change in that induced field on one encounter. That was because you get point samples of the magnetic field, and by unhappy luck all of their flybys occured when the field would have rougly the same alignment. If you put a probe in orbit, you can really milk that information as the moon moves through the Jovian magnetosphere.

All the other investigations can be done, if not as well, using multiple flybys. But those two investigations might prove to provide some of the most important revelations.

JSO would undoubtably add considerably to our knowledge of the whole Jovian system. But that might be all it did: be a super-Galileo. Might not be a lot of paradigm shifts.

EE would at least be an enhanced-Galileo with it's Callisto and Ganymede encounters. But in the case of Europa it has the likely potential to uncover very fundamental disocveries that would completely revolutionize our knowledge of Europa.

So in my mind, if you're going to spend roughly the same ammount of money for JSO vs. EE, I'd go for EE.

You could make those same arguments regarding the uniqueness of Europa with just about any other body in the solar system. You can make it about Io, Enceladus and Titan in this context without any exaggeration.

Doug

I think that the additional time observing the Jovian system is a big selling point, particularly if it has a good long range camera. Another factor is that JSO, which will not have to deal with the Jovian radiation at Europa for extended periods of time, may not be as technically difficult, which might mean that it may be less likely to be delayed. Another factor is that once it is in Ganymede orbit, it can be used to the last drop, because it doesn't pose nearly the planetary protection issue a Europa orbiter would. I do think the informed public does like Europa. Still, I think they could be sold on the frozen tectonics of Ganymede (which could be related to the early earth - a stretch, but so are Europan brine shrimp) and the volcanoes of Io, not to mention Jupiter.

Speaking of that, we do have one more mission on deck already, Juno. Granted, it won't be much for image hounds and moon-lovers. Still, having an outer planets mission in queue is a good thing, because it keeps the bean counters used to having one in development. Continuity is an important thing.

This is really the problem that I am having with the EE. When you compare the general features of the tours for EE and JSO (especially before moon insertion), EE comes up short. Because of the more intense radiation, EE's shielding must be better such that its science payload is significantly reduced--JSO can carry 40% more instrumentation than EE. All the instruments on JSO seem much more capable.

JSO's tour is much more balanced as well. We get to enjoy a full three years during the Jovian phase for JSO, while the current tour for EE is only two years. As also mentioned, Io, Europa, Ganymede, and Callisto all see close flybys before GOI (currently 4, 6, 7 and 11). Moon orbit insertion should certainly be within our technological limits, but if something were to go wrong, at least we would have good science for the entire system. This reduces some of the risk. EE is much less varied and skips Io altogether (currently 0, 4, 13, 4). If there were a critical failure during EOI, then EE would actually return very little Europan science. (Would it then be known as GE?).

I tend to agree with Ted that the longer observation time with JSO, including a significantly longer life after GOI, is very appealing.


I would have to strongly disagree with this statement. Sure, from a Fe-rich or silica-rich magmatic point of view, the Europan surface has little to offer. Differentiation occurred long ago, sending all of the heavier minerals into the core. But there aren’t many craters on Europa—its surface is probably younger than much of Earth’s surface. From a tectonics point of view (which is very much in the realm of geologic interest), this is a very interesting problem that should be investigated.

For me, it’s this surface activity (and not the astrobiology) that catches my interest. Sure, Io would be more desirable, but it isn’t feasible. In the end, I have to prefer parking the probe around Europa, but I can’t support the current pre-EOI tour. How firm are these EE tour plans? Some sort of hybrid must be possible.

Sorry if I was unclear on the choice I was making. I was comparing JSO vs. EE.

Many (but not all) of the arguments on this thread were boiling down to having Europa vs. Ganymede as the final destination of the probe. In my mind if your choice is between orbiting Europa, or Ganymede, then Europa wins.

I was not even considering or discussing Io, Enceladus or Titan.

The swing argument in favor of making the next Flagship mission the Europa Explorer may be preparation for follow on missions. We won't be able to answer any biological questions about Europa with EE -- but we have to find the safest and most scientifically promising landing site on Europa for that mission, which is in fact the central justification for Europa Explorer. Europa landers -- like Mars sample returns -- will be very few and far between, and we have to take great care to maximize both the chances that they'll survive and the chances that they'll find something really interesting. Without that need, Europa Explorer's high-priority status really would be questionable -- but that need is the elephant in the room, assuming that we want to have any chance of examining Europa for life before about 2040. Don't forget that we are talking about flying only one of these Flagships every 8-10 years.

Possible compromise: the Europa Explorer report mentions (pg. 160) that delaying its launch by 19 months -- till Jan. 2017 -- not only allows its funding to be more stretched out, but also provides a considerably better launch opportunity that would allow the craft's total mass to be increased by about 550 kg, and its dry mass to be increased by 175 kg. You could use that extra mass for more and better instruments a la JSO, and/or for more radiation shielding and fuel to allow a few Io flybys (in addition to the already-existing Ganymede and Callisto flybys) before settling into Europa orbit. This, after all, is the launch date already labelled for JSO. (Also keep in mind that in the two studies, JSO cost about the same as Europa Explorer. That's not surprising, given that their designs are near-identical except that JSO would carry more instruments and can compensate for that by having modestly thinner radiation shielding -- although it must still survive 1.8 Mrads versus 2.6 for Explorer.)

That's an interesting argument well worth considering from many angles. Does anyone know of anything else planned long-term for Jupiter other than Europan exploration? (I don't.) This may reflect a fundamental perceptual bias on our part, and we probably wouldn't be having this discussion if Galileo's HGA had fully deployed, alas and dammit.

IMHO, Ted is right: we need to complete a detailed reconnaissance of the Jovian system before committing to target-specific Flagship-class missions. Europa is very attractive, of course, and think it would be the proper EOM objective for a future systemic orbiter...but we need more data. We may be fixated on Europa to the detriment of considering other, potentially more scientifically significant, objectives. Point being, we don't have a very clear picture of the Galileans yet...might be wise to acquire one before proceeding further.

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...