In about three weeks, we'll know which outer planets flagship mission -- Titan-Saturn or Europa-Jupiter will be chosen.
Right now, the voting on the blog is running 74% in favor of Titan-Saturn and 26% in favor of Europa-Jupiter.
My preference seems to change daily and it is always close. These are excellent proposals and both deserve to fly. They are also highly complimentary since they explore moons of gas giants. Titan's geology likely will be harder to decipher because its surfaces is modified by atmospheric processes (what lies beneath all those dunes, for example?). The Jovian moons lack an atmsphere to modify their surfaces and can provide examples of processes that are likely to operate on Titan but that might be obscured.
Much of the decision process that NASA and ESA will use will revolve around details of mission risk and technology readiness that the public will not have knowledge of. We therefore cannot handicap that portion of the decision making.
Here are some personal observations I keep in mind when trying to decide between the two missions:
As a mission to explore a gas giant system, I think that Europa-Jupiter gets the nod since it will include observations of 4 moons, the magnetosphere, and Jupiter itself. The Titan-Saturn mission appears to be more narrowly focused on just two moons although it will also include magenetosphere studies. The strength of the Europa-Jupiter mission as an exploration of the Jovian system would be enhanced if Japan flies its proposed magnetosphere orbiter.
As a mission to explore individual moons, I think that Titan-Saturn gets the nod since it can sample the interior material of Enceladus by flying through the plumes and can land on Titan and float in its atmosphere. (The Europa-Jupiter mission will sample material sputtered off the moons, but I think the concentrated and probably relatively unaltered material from Enceladus is a better sample.)
One of the goals of the Europa orbiter is to find one or more locations where a future lander could sample relatively pristine ocean material that lies at or near the surface. If no locations are identified, then the exploration of Europa may be stymied. If one or more locations are found, the next step of Europan exploration would require a sophisticated lander (and possibly radiation hardening technologies that don’t yet exist) that I suspect would be Flagship mission in its own right. Such a follow on mission would have to wait the results of the Europa orbiter, and so probably wouldn’t launch until the late 2030s or 2040s. Russia is exploring the idea of launching a Europan lander which would address at least part of this issue (how much depends on how capable the lander would be).
Based on previous studies of missions, it appears to be cheaper to make up a meaningful portion of the Europa-Jupiter mission with cheaper mission alternatives than with the Titan-Saturn mission. Titan and Saturn appear to require Flagship scale missions while the Jovian system can be explored (IN MUCH LESS DEPTH) with New Frontiers and maybe even Discovery missions.
The ESA contribution to either mission is not guaranteed. ESA will decide between participating in the the winning Flagship mission and other missions proposed for its Cosmic Vision program in 2011. It is always hazardous to second guess mission selection processes (my record would be no better than if I'd thrown darts), but I have a feeling that the Titan lander and balloon might fare better in the selection than a Ganymede orbiter. Titan builds on ESA success and expertise in Titan probes from Huyegens. The public appeal of a lander floating on a sea on another world and a balloon floating above an alien but strangely Earth-like world be tremendous.
I'm sure that there are considerations that I haven't thought of; feel free to list them in the comments. Whichever way it goes, an excellent proposal will be selected and an excellent proposal will be rejected.