As with all my editorials, I am not trying to convince anyone to my point of view. No one on the Decadal Survey has ever heard of me, and my opinion counts for no more than any of the readers of this blog. Rather, I find that reading a good editorial (and I hope my efforts are 'good') helps me focus my own thinking and reach my own conclusions. So, with that in mind, here is one of the rare editorials on this blog.
In the next few months, the Decadal Survey will have to select missions to recommend for flight in the coming decade (2013-2022). We have already been warned that sticker shock is coming and that fewer missions can be flown than advocates and scientists would hope. At $3.2B and perhaps $4B with inflation and cost increases, the Jupiter Europa Orbiter would consume a substantial chunk of that ~$12-13B budget. (ESA's Jupiter Ganymede Orbiter, if selected, would be paid for out of Europe's budget for its next large science mission.) Perhaps most damning, funding EJSM would preclude funding for a flagship mission to Titan and Enceladus.
My first reason for favoring EJSM is that it would explore three classes of important objects: (1) icy moons that may be habitats of life either in our solar system or others, (2) a large gas giant that is our best analogue for the many gas giants found around other stars, and (3) an intense magnetospheres that serves as a surrogate for other such structures in the universe. The last two points speak for themselves, but I will expand a bit on the first point. I believe that the ultimate exploration of an icy moon environment will be at Titan, but that world has such active surface processes that untangling its geologic history will prove difficult. At Jupiter, we have four moons that provide case studies a range of tidally influenced moons (with Io and Callisto at the extremes) without the confusion of active surface processes. Finally, Europa may be a habitat for life, and we should explore that potential with a highly capable spacecraft.
My second reason for favoring EJSM is that the JEO mission is ready to fly. A decade of technology development and mission design has brought the mission to a point where risks are low. We learned last year in the shoot out between EJSM and the Saturn Titan System Mission (TSSM) that the same was not true for concepts to explore the Saturn system (click here for the orbiter and here for the in situ elements).
My final reason for favoring EJSM is the lesson learned from Mars exploration in the last decade. At the Red Planet, we have learned that a series of highly capable missions can together bring a deep insight into a world or, in the case of Jupiter, a system of worlds. NASA's JEO could be just the most capable of a fleet of craft that could also include ESA's Jupiter Ganymede Orbiter, Japan's magnetosphere orbiter, Russia's Europa lander, and possibly penetrators for Ganymede and/or Europa from another space agency. Together, this flotilla would do for the Jovian system what a decade of missions have done for Mars. What we learn from the Galilean moons will build towards our understanding of ice-ocean-rock moons including Titan and Enceladus.
Of these three arguments, I personally find the final most compelling. We have done most of the easy missions for the solar system. Significantly deepening our understanding of key worlds and systems will require focused exploration. (Even if JEO turns out to be the only mission to fly to Jupiter, it is capable enough that it would count in my opinion as focused exploration.) In the coming decade, I favor focused exploration on three and a half targets. First, there will continue to be Mars which is likely to receive several orbiters from Russia, China, and ESA/NASA, 2-3 rovers, and possibly a network of science stations. Second, could be the Jovian system. And third, there could be Venus which could be the recipient of Russian and American landers, a European balloon platform, and several orbiters. All in all, the next decade, thanks to the combined contributions of a number of space agencies, could see the in-depth exploration of the solar system expand from one target (Mars) to several.
The half target in my scenario would be the Saturn system. Eventually, we need to return there with flagship class spacecraft. I found the case laid out for a flagship class orbiter to take the global study of Titan to the next level in the TSSM study compelling. Not only will a battery of instruments be required, but a high power communications system (which drives the need for a flagship class spacecraft) is essential to return the data stream. However, there are, I think, a couple of low hanging fruits available for the Saturn system. The first is Enceladus, for which a New Frontiers-class mission with with advanced instruments should provide a significant advancement in our understanding. The second is in situ probes for Titan, which is about the easiest place in the solar system to land on or fly or float above. The proposals in progress for a Discovery-class lake lander and a Discovery-class airplane suggest that in situ Titan probes could be within the budgets and technical capabilites of several space agencies in the coming decade. The key problem for most in situ probes is the data communications challenge -- there simply isn't room within these probes to house the power systems and antennas to return large amounts of data. So, I favor a New Frontiers class orbiter that would switch between focused Enceladus studies and relay duties for Titan in situ probes over the course of perhaps a decade or more in orbit around Saturn.
For the past year, I have closely followed the Decadal Survey process as well as the planning processes of other space agencies. In this blog entry, I lay out the conclusions I've reached. I hope that the readers of this blog will lay out their own or challenge mine in their comments.