With that background, I watched the Decadal Survey process as both an interested citizen and as a former practicioner of strategic planning. I came away impressed with both the process and the results. A measure of the thoroughness of the process is we have a plan that holds up well despite a large cut in projected funding that occured during the process.
Still, the process did have some underlying assumptions that changed with projected funding cuts. One assumption was that a scaled back Europa orbiter mission (JEO) teamed with ESA's proposed Jupiter Ganymede Orbiter (JGO) might be funded in the next decade. The former now looks unlikely and the latter is in competition against two other good proposed missions. Because of those assumptions, the survey assumed that the planetary community's highest priority icy-ocean moon mission might fly. As a result, the Survey members did not pursue studies of a minimum cost Europa missions or of low cost Titan multi-flyby or orbiter. The Survey included a study for a Ganymede orbiter and concluded that, "the key required Ganymede flight system elements are being developed and demonstrated for Jupiter applications on the Juno mission," and "a Ganymede orbiter mission appears to be technically feasible with no required technology development," but, "the mission was not given further consideration because of the likelihood that the ESA Jupiter Ganymede Orbiter would achieve most of the same science goals." (All quotes in the post are from the Survey report.)
The committee did forsee that JEO and JGO might not fly in the coming decade, and recommended a Flagship-class Uranus orbiter and Enceladus orbiter missions as backups to provide compelling new science on icy moons (although the Uranus orbiter's study of the moons might have been limited). The report noted that, "In particular, because of the broad similarity of its science goals to those of JEO, NASA should consider flying the Enceladus Orbiter in the decade 2013-2022 only if JEO is not carried out in that decade." In the current budget situation, it seems likely that neither of these missions as proposed are likely to be funded, either.
So, at the moment, the study of icy-ocean moons depends on JGO winning its selection competition and/or the Discovery Titan Mare Explorer (TiME) winning its selection competition. That raises the question of whether NASA should at some point reconsider New Frontiers class missions to the icy moons of Jupiter and Saturn.
NASA may have a partial answer, but not be able to share it with the public. At least two multi-flyby outer planet moon missions were proposed for the current Discovery selection: the Journey to Enceladus and Titan (JET) and an Io mission. Part of the review process would have been a technical and fiscal feasibility assessment. If these missions were judged feasible, then the Discovery program would be one mechanism to continue the exploration of the outer moons. (NASA, quite correctly, keeps the reviews confidential and shares them only with the proposers.) The fact that neither of these missions made it to the final candidate list should not a priori lead to the conclusion that they were weak proposals. TiME is an strong proposal that, as I understand it, must be selected in this competition to launch by mid decade to reach the northern lakes of Titan before the changing seasons hide them from direct communication with Earth.
NASA and its planetary science community may conclude that a New Frontiers class mission may still be desired, either because Discovery mission are not feasible to continue the exploration of the outer planet moons or to enable more complex missions. Among the missions that might be considered could be:
- A Ganymede orbiter with multiple flybys of Callisto and perhaps Europa if ESA does not select JGO
- A very scaled back Europa orbiter (although these feels like a stretch to me within a New Frontiers budget)
- An Enceladus and Titan multiflyby mission. (Several versions of this type of mission were considered by the Survey, and one had possible costs that were not too far outside the budget for future New Frontiers missions.)
As a former practitioner of strategic planning, I am not one to call for changes to a just published plan -- that defeats the entire purpose of a rigorous process. Can there be a middle ground that doesn't immediately reopen the recommendations of the Decadal Survey yet can allow flexibility?
Following the previous Survey, an interim committee met several years later and recommended adding to the list of proposed missions to be considered for New Frontiers mission. A similar process in the middle of this decade has been speculated about in some of the meetings I've listened to.
So what might the criteria be for considering adding a Jupiter or Saturn icy moon mission to the list? Perhaps the process might be something like this. First, wait and see how the JGO and TiME missions fare, how icy-moon missions fare in future Discovery mission selections, and how budgets actually develop. Then, if a New Frontiers mission to these moons still is compelling, commission studies to develop the concepts and confirm their costs using the same rigorous process followed for the Decadal Survey. If the missions appear technically and fiscally feasible, then use a panel of senior scientists to revisit the list of New Frontier candidates for the second selection near the end of this decade. (The current competition with OSIRIS-REx as the selected mission, was NF 3; NF 4 would be selected mid-decade followed by NF 5 late this decade or early in the next.) If an outer planets mission were to be added, then either the Io multiflyby or Saturn probe mission might be dropped to keep the candidate list balanced among the types of solar system missions. (In this spirit, the near Earth asteroid sample return mission was to be dropped for the NF 4 and NF5 competitions regardless of the NF 3 selection because the Trojan asteroid tour and rendezvous mission was added to the list to reflect the priorities in the primitive bodies community.)
In a nutshell, allow events to play out and if a revisit of the list of candidate missions appears appropriate, follow a rigorous process that the entire community would see as fair and considered.