OPAG Reports, Formal proposals/evaluations of future outer SS missions Options

[ngunn]

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.

[nprev]

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?

[tedstryk]

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.

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.

[JRehling]

Interesting question. I think there is a lot of great science to be had from planetary monitoring
[...]
But ... NASA is in the business of building spacecraft rather than infrastructure.

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.

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.)

(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)

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.

[nprev]

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.

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.

[tedstryk]

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).

[ugordan]

Galileo's coverage was so spotty that you can't convince me that there aren't new major areas of interest we are missing.

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.

[JRehling]

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.

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!

[volcanopele]

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.

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...

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 a trendy thing to want to understand Europa, the moon's under the microscope for a good reason IMO.

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.

[ugordan]

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.

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.

[nprev]

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.

[vjkane]

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.

[tedstryk]

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.

[nprev]

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.

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!)

[tedstryk]

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.