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

[ngunn]

but is Ganymede really that worthy of a target?

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.

[nprev]

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.

[ugordan]

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.

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"?

[nprev]

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.

[volcanopele]

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...

[ugordan]

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.

[vjkane]

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.

[Juramike]

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

[Mariner9]

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.

[djellison]

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

[tedstryk]

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.

[NMRguy]

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.

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.

relatively boring ice geology

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.

[Mariner9]

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

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.

[vjkane]

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.)

[nprev]

The swing argument in favor of making the next Flagship mission the Europa Explorer may be preparation for follow on missions.

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.