In a previous post, I suggested that a Galilean satellite observer might be a potential fallback if the Jupiter Europa Orbiter is not funded. Such a mission would extend the goals of the proposed Ganymede Observer by visiting additional moons. I previously posted a response from Bruce Moomaw.
John R., who always seems to have thoughtful responses, sent the following response to my proposal, which I publish with his permission:
The perfect is the enemy of the good... and vice versa. The question is which to root for.
In this case, I'd hold out for perfect. An orbiter that has half the cost has to have more than half the scientific value, and this wouldn't cut it.
In my opinion, any jovian mission that doesn't orbit Europa has to play some very distinct role for relatively cheap (eg, Juno, much cheaper than JEO), or I'd recommend tabling the jovian system indefinitely. Aside from Juno, the only cheap alternative I could see would be something that stays totally out of the radiation belts for most of its main mission and observes Io over long periods of time with incidental observations of Callisto and Ganymede up-close. Maybe finish the mission with some deep-dives that reveal Io closely with passes by Europa along the way. But only if this is as cheap as Juno.
The way I see it, we're engaged in expensive games of Twenty Questions with Venus, Mars, Europa, Titan, Enceladus, Neptune/Triton, and other targets, with varying levels of priority and difficulty. We're necessarily going to be ignoring some of these targets. I'd have no sacred cows about which avoid being ignored and emphasize that every question *counts* -- maximally. Any place where the funding climate (or other factors) preclude us from asking a question that is worth the expense, we should sadly put on the Ignore list and let something else percolate up. Venus has taken the interminable delays for quite a while now. Mars even experienced that. It's going to happen to all of them.
Europa is actually the one place on the list where it might be most certain what the next mission should be. (Mars is a bit clouded by the variety of options; Neptune also seems to have some clarity about it, but loses out in terms of priority/difficulty.) Europa would be the one place where I'd most hate to have us satisfice.
Venus and Mars lend themselves most easily to satisficing; we can isolate individual scientific questions for a fixed price without making a massive commitment. My sense of the big picture would be to say that JEO, a modest mission to Venus (atmospheric probe that definitively nails down the isotopic abundances and does spot observations of the surface), and whatever comes next at Mars (a big issue in itself) are the *only* options I'd put on the table for major missions, and I'd drum my fingers on the table and take longer development periods towards fewer launches until those move through the queue.
The beauty of the Twenty Questions approach is that it can lend clarity to what comes next. It could tell us that Europa might merit hogging the pipeline for a while, or that it should be bumped far down the list. If we only observe 15% of Europa's surface closely, we might remain as uncertain as we are now.
Any Galilean [observer] mission might delay an eventual JEO. I don't find any intrinsic value in science at Target X over Target Y such that we need to visit Target X on a timely fashion to keep the pipeline going. If a mission has half the cost of JEO and less than half the science value, then I would NEVER fly it, ever, unless the rest of the solar system utterly lacked for alternatives (which isn't apt to be anyone's conclusion). I would fly fifty consecutive missions to Mars, if that's what it came to, rather than one to the Galileans that doesn't set up the next Galilean mission. Since JEO planning is mature, I would never fly any mission that is preparatory to JEO unless it had a clear cost/benefit advantage. Eg, if it returns 25% the science value, then it has to be under 25% the cost. Which seems likely to be impossible. So I'd never fly Galileo II.