I've been following the Survey's progress probably as closely as anyone outside of the process has. During the last few months, I've been wondering what criteria I would use to set priorities. And that led me to thinking what priorities would I choose.
I want to emphasize that what follows isn't an attempt to persuade anyone about what the priorities should be. Your opinions are as valid as mine, and except for perhaps a lucky one or two readers who may be involved in the process, your and my opinions are likely to have the same impact. However, I often learn more from a carefully crafted argument than I do from just reading the facts (which is why I read the opinions page of the newspaper more regularly than the front page). So here goes (and I hope that you find this carefully crafted).
A successful program has to meet a number of goals:
- It must be fiscally possible. That is, it has to fit within the $12-12.5B budget expected for the next decade.
- It must be compelling to the public and to the politicians who will have to prioritize dollars spent here over other worthy projects over the course of a decade.
- It must significantly advance our scientific understanding of the solar system.
- It should provide a balance between types of solar system bodies. The list of discipline panels suggests the breadth possible: Inner planets, Mars, giant planets, outer solar system satellites, and primitive bodes
There's another criteria that may or may not be adopted by the Survey, but I think should be. It is quite possible that over the course of the decade that the budget for planetary exploration may be cut. (Recent reports that NASA will not receive a $1B budget boost for the next fiscal year leads credence to this fear. Planetary missions will be in competition with the politically popular manned and Earth observation programs.) I believe that the prioritized list of missions should remain useful if budget cuts occur.
I think that the most critical decision the Survey will face will be setting the balance between the large Flagship missions and smaller New Frontiers and Discovery class missions. Two Flagship missions are likely candidates, Mars Sample Return (MSR) and the Jupiter Europa Orbiter (JEO). These missions are large enough that they together would chew up three quarters of the budget. However, it could be argued that these two large missions would provide the greatest scientific return of any missions on the candidate list. There would also sufficient budget to fly the Mars Trace Gas Oribiter, support the ExoMars rover (as part of MSR's MAX-C rover), and fly three smaller missions which could visit the inner planets and primitive bodies (MSR and JEO would take care of Mars, the giant planets, and outer solar system satellites).
Example of a priority list and budget for a program that emphasizes Flagship missions. Figures are in $Bs, and use either published estimates or my own best guesses. New Frontiers and Discovery figures include the PI budget (~$650M and ~$450M) plus launches and other overhead.
I have two concerns about this priority list. First, MSR and JEO could easily experience large cost overruns that prevent missions to other destinations. Second, large missions make tempting targets for politicians looking to cut budgets. Both of these missions would have to find political support across at least two Presidents and about a half dozen Congresses. (In America, we elect the entire House of Representatives and a third of the Senators every two years, which counts as a new Congress (even though the reelection rate is so high that 'new' may be a misnomer).) The counter argument is that scientific support for these big missions would be great enough to shield them from cancellation. There's some truth to this (look at the survival of the James Webb telescope and the Mars Science Laboratory despite massive overruns). On the other hand, a number of large science programs have been cut in the past; remember the Comet Rendezvous and Asteroid Flyby (CRAF) sister ship of Cassini cut in the early 1990s.
I instead prefer a priority list that focuses on smaller missions to many solar system targets. Then if budgets permit, I would fly the full JEO mission and the first component of MSR, the MAX-C rover, as the lowest priorities. This way, if budgets are cut, there is still a robust program of missions to a number of destinations.
How I would prioritized my list of missions based on mission concepts that have been studied to date.
While this list may seem like a scattered set of missions, there are some themes. Mars remains the top priority with over a third of the budget. Icy moons would be a priority with missions to the Jovian moons, Titan, Enceladus, and Triton. The single Venus mission looks lonesome, but I see it as the NASA contribution to an international flotilla to that world. However, the primitive bodies scientists would rightfully feel left out with just a single Discovery mission. (I left both the Venus and primitive bodies missions as placeholders. Depending on which missions are selected in the current New Frontiers and Discovery competitions, the priorities for these missions may change.) It was really hard leaving off some missions I really like and that would produce great science like the Io Volcanic Observer (IVO). (However, I could imagine a scenario where a line of New Frontiers class spacecraft were built using nearly identical spacecraft for the JMO, TEO, and Argo missions and then the savings might fund IVO.)
You'll note that I don't specify specific targets for several Discovery and New Frontiers class missions. I have concerns about the totally open competitions to date, which make it impossible to know which missions will fly. A consequence of this is that NASA has a harder time prioritizing its technology development funds. If, however, NASA knows that it will be flying a lander to Venus, it can prioritize the appropriate development work. Prioritizing specific targets also makes it easier to put together international collaborations. If NASA will be flying a Titan/Enceladus observer, then perhaps another space agency would fund a Saturn atmospheric probe or a Titan lander to ride along.
As I said in the beginning, there's nothing special about my list. However, I hope that it will get you to thinking about what priorities you would set.
Oh, and as the Survey studies additional mission concepts, it is likely that my list of priorities will change. There's a number of exciting concepts under study.