A few blog entries ago, I gave a summary of tidbits from the recent EJSM instrument meeting. As part of that entry, I reproduced a chart (shown below) that showed what kinds of science might be done from a Galilean moon observer (GMO) that conducts multiple moon flybys but orbits none. This could be a fall back plan if the Jupiter Europa orbiter (JEO) proves too expensive at $3B+.
Bruce Moomaw wrote me to remind me that a GMO would seriously compromise the goals for Europan exploration:
"The trouble with substituting multiple Europa flybys for a Europa orbiter is simply that the single most important task for the Europa Orbiter -- beyond confirming that Europa does have a liquid ocean and might be habitable -- is to find the best possible place for followup Europa astrobiological landers, both in scientific value and in safety. And you've seen for yourself from that chart how extremely little of Europa's surface would be surveyed by 6 flybys. If we don't launch a Europa orbiter in the 2020 or so period, we will definitely have to launch one later in any case to survey Europa. (There are analogies with the need to fly one or more organic-detecting Mars rovers before we fly a Mars sample return.)"
I fully agree with Bruce that Europa should be orbited. However, wishes don't always come true. The United States is saddled with large and growing budget deficits. NASA's manned spaceflight plans are seriously underfunded. Our home planet needs more attention from a new generation of Earth observation satellites. And short of Congress adding significant money to NASA's planetary budget, we can't afford both a JEO and an aggressive Mars program. So JEO may never fly. A GMO, though, might be possible at ~$1-1.5B, or the price of a mid range Mars rover.
Observer missions to Io and Ganymede have been made priorities by NASA's scientific advisory boards (and both are possible targets of the next New Frontiers or Discovery selection, although the selection of JEO as the next Flagship mission may make this opportunity mote for these mission concepts). If JEO doesn't fly, then it's not hard to imagine that a Europa observer would make the list.
What might a GMO mission look like? I'm not a mission planner (although that would be a hell of a fun job). Here are some ideas, though:
The tour begins with a small number of Io flybys
The next target would be a series of Europa flybys that would pass low over the surface so that the ice penetrating radar can sample ice depth at several locations.
Following Europa, the spacecraft moves out of the high radiation belts and begins a campaign of Ganymede and Callisto flybys.
In an expected mission extension, the spacecraft returns to flybys of Europa to extend high resolution mapping (perhaps with a different side of Europa illuminated) and more probes of the ice depth.
I believe that if more money was possible (but still not enough for JEO), I think that a dedicated Io observer would be a good bet. A second craft would eliminate the need for GMO to be exposed to the intense radiation surrounding Io. A Io observer could also carry instruments optimized to measure the surface, atmosphere, and possibly plumes of Io. This second craft would also be in a polar orbit around Jupiter, allowing complementary magnetosphere measurements with the equatorial GMO orbit.
A mission focused on satellite flybys would undoubtedly be optimized to provide more high resolution coverage of the moons than listed in the chart above, especially for the ice penetrating radar at Europa. From a flyby mission, we would likely learn whether the average depth of Europa’s crust is thin (which would make life much easier since organic molecules produced on the surface could more easily migrate into the ocean) or thick or a heterogeneous mixture of thicknesses. The biggest loss would be that we would likely not discover locations – if they exist – where the crust is much thinner than is typical. That science requires an orbiter. I hope one flies in my lifetime (which should last another 30+ years). If not, I’d rather see an Galilean satellite observer than no mission to these moons at all. I would make the same point about a follow up mission to Enceladus and Titan.