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.

[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]

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!