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

[JRehling]

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.

[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.

[centsworth_II]

...we probably wouldn't be having this discussion if Galileo's HGA had fully deployed...

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.

[JRehling]

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.