The presentations at the February OPAG meeting held statements that the Europa versus Titan decision may come back up for review. Last year, two teams competed to be selected for the next NASA Flagship mission and for a place in ESA's next large mission competition. The review teams concluded that the science from either mission would be equally good, but that a decade of technology development made the Europa mission much more ready for development than the Titan mission. A NASA Europa mission got the nod and an ESA Ganymede orbiter won a spot to compete against two astronomy missions (with the ESA decision to come next year). Both space agencies promised to fund advanced mission planning and technology development to make the Titan mission ready for its own start later in the decade. [Editorial note: The Survey could decide not to prioritize missions to either Europa or Titan, but I wouldn't take a bet on that outcome.]
Now, that decision on the American side is up for review as part of the Decadal Survey. NASA has made it clear that it will take its priorities for planetary missions in the coming decade from the Survey's priorities. Any mission that has not received a formal new start from Congress must be recommended by the Survey for NASA to procede. That includes the Jupiter Europa orbiter.
NASA's current budgets continue to fund the Jupiter-Europa for Phase A, which is the period of advanced development before final design, manufacture, and testing begin. Funding beyond Phase A is contingent on the Decadal Survey making this mission a priority. To do that, the Survey would have to deprioritize other elements in NASA's current roadmap. At $3.2B, there simply isn't room in the budget for this mission and the current Mars program and the New Frontiers and Discovery programs (see here for analysis of latest NASA budget proposal).
At the same time, the proponents of Titan as the choice for outer planet exploration are working to be making their voices heard. One of them, Ralph Lorenz, presented at the meeting. He showed that Titan has been to subject of many more scientific articles than has Europa over time. (To be fair, the Cassini-Huygens mission has returned far more data on that moon than the crippled Galileo spacecraft did for Europa.) He also complained that promised funding for advanced development of a future Titan mission has not been forthcoming. As an example, he described how promised money to develop balloon technology for a Titan mission had gone instead to fund the Decadal Survey.
Cassini's continued exploration at Titan continues to build the case for returning to that world and to neighboring Enceladus. The case also is being made that Titan could be home to exotic forms of life and one scientific paper has prioritized Titan ahead of Mars as the best place for astrobiological exploration in the solar system (Europa came in third). Recent finds at Enceladus continue to build the case that that moon has an internal ocean that could host life.
Whether to return to Europa or Titan or both or neither in the coming decade looks to be one of the biggest decisions facing the Decadal Survey.
Editorial Thoughts: Europa-Jupiter and Titan-Enceladus-Saturn both are compelling targets for exploration. Both destinations should be explored in the coming decade. However, the radiation belts at Jupiter impose significant technical challenges to any mission that will meaningfully answer the question of whether Europa could be explored for life, either by finding a thin point in the ice to penetrate or by finding a location where recent eruptions have brought ocean material to the surface. On the other hand, Titan-Enceladus-Saturn presents a relatively benign environment (if a bit chilly within the atmosphere of Titan), but the mission design and technology apparently aren't ready to fly for some of the most exciting mission concepts.
If the decision were up to me, I would commit to the $3.2B Europa-Jupiter mission and budget another $2B for Titan-Enceladus-Saturn, which might be expanded if other space agencies contributed. I'd go ahead with the Jupiter Europa Orbiter as planned -- it's ready to go and the harsh environment at Europa doesn't favor cheap missions. It would also do for the Jupiter system what Cassini is doing for the Saturn system and what Galileo with its crippled antenna couldn't. Around $2B probably would fund a highly capable orbiter or a less capable orbiter (perhaps similar in scale to the proposed Io Volcano Observer) and a Titan in-situ probe such as a lake lander.
An alternative strategy would be to commit ~$3B to Titan-Enceladus-Saturn, perhaps for a combination of New Frontiers scale missions. Perhaps a Saturn orbiter could execute a number of flybys of Titan and Enceladus with instruments tuned to fill gaps in Cassini's investigations. The orbiter could also act as a relay for one or two in situ Titan craft, perhaps the Titan Mare Explorer and the aerial AVIATR. The Europa-Jupiter mission would then be constrained to a $2B mission. Within that budget, a capable craft could perform a number of flybys of all the Galilean moons, study Jupiter from afar, and perhaps orbit Europe for weeks instead of the months planned for the Jupiter-Europa Orbiter. However, the orbiter missions under investigation by the Decadal Survey do not currently include alternatives to the Flagship, ~ $3B, Europa Jupiter System Mission and the Titan Saturn System Mission for exploring those two moons. The Survey is looking at a number of mission concepts to explore Enceladus, a Titan Lake lander, a Ganymede observer, and an Io observer.
Either of these plans could leave ~$5B for Mars exploration, which would fund the Mars Trace Gas orbiter and the 2018 ExoMars/Max-C mission and leave ~$2.5B for other Mars missions, presumably down payment on a sample return. In this scheme, lunar, inner planet, and small body exploration would have to share the remaining ~$2B, which would fund perhaps a Discovery mission and a New Frontiers mission.
Baring breaking news on future planetary exploration, the next two blog entries will look at the planning for the science that the Jupiter Europa Orbiter could do for Jupiter and the Galilean moons other than Europa.
Updates on NASA outer planets program and status of Jupiter Europa Orbiter funding from Feburary OPAG meeting