Spaceflightnow.com had a recent article on the competition between Mars sample return and the Jupiter System mission for the next large NASA and ESA missions. (The article primarily focuses on the Mars mission, but I'd like to explore the competition.) Both missions are large: The sample return is likely to cost $6-7B while the combined costs to NASA and ESA of the Jupiter Europa and Jupiter Ganymede missions will be ~$4.5B. The backers of a Mars sample return would like to see the three elements of the sample return fly in 2018 (the ExoMars rover and the sample cache rover, MAX-C), 2022 (an orbiter to carry the samples brought up from the surface back to Earth), and 2024 (a lander and ascent vehicle to deliver the sample to Mars orbit). (I believe that the 2022 mission would have to enter development around 2018 in this scenario.) The backers of the Jupiter system mission would like to see that mission fly in 2020.
The Spaceflightnow.com article and others (see Decadal Survey Showdown) have suggested that the two space agencies cannot afford both sets of missions. The U.S Decadal Survey is seen as the body that will decide between the missions. In this blog post, I consider whether it might be possible for both to fly without consuming the budget.
The ESA elements for the 2018 ExoMars rover are already budgeted, while ESA's Jupiter Ganymede orbiter (if chosen over two other missions) would be funded out of its large science mission budget. Funding for subsequent ESA contributions to the sample return, would according to the Spaceflightnow.com article, require more money than is currently being budgeted for Mars exploration.
On the NASA side, there is room in a ~$12-13B (in current dollars) decade budget to fly both missions. Doing so would consume most of that budget, probably crowding out all but a handful of low cost missions to other targets. (And if these two large missions experience significant cost overruns, even those small missions may have to go.)
However, I wonder if there might be a sequence of missions that allows both to fly. The currently discussed sample return mission series assumes that missions fly as quickly as budgets could allow. Another option would be fly the 2018 sample mission and wait for its results to commit to the subsequent orbiter and lander/ascent vehicle. The 2018 rover might crash on landing, become stuck in the mother of all dust pits for eternity, or simply fail to find compelling samples. Committing to flying the subsequent sample return elements before the results of the 2018 mission are known would seem to commit NASA's resources to a single large program predicated on success in 2018.
An alternative might be to fly the sample cache mission in 2018 and the Jupiter Europa mission in 2020 (hopefully with their ESA counterparts). The results of the 2018 mission should be known by 2020 or 2021, and then if compelling samples are safely cached, development of the subsequent missions could begin with flights in the later half of the 2020s. (I don't know whether a decade or more waiting on the Martian surface would cause the cached samples to degrade.) In this scenario, the NASA would need to commit $6-7B between the two missions, leaving room for a couple of New Frontiers and Discovery missions. The majority of the remaining funds needed for the subsequent sample return elements would come from the following decade's budget.
It's easy to be an armchair mission planner. There may be many reasons why this possiblity may be infeasible or unwise and there may be better alternatives. The Decadal Survey may also conclude that it wants to recommend just one (or no) multi-billion dollar missions in the coming decade, eliminating the competition. However, I have not seen a discussion of the question of whether or not results from the 2018 caching mission should be known before committing to the next sample return element. I hope that the Decadal Survey will consider that question as it reportedly is considering whether to recommend either or both of these two large missions.