The best hope for NASA missions to the icy-ocean moons of Jupiter and Saturn in the coming decade may lie in the low cost Discovery mission program. In past posts, I've described two proposals that I understand were proposed for the current Discovery mission selection competition: the Titan Mare Explorer (TiME) lake lander and the Journey to Enceladus and Titan (JET) multiple flyby mission
In addition, I understand that the Io Volcano Observer (IVO) multiple flyby mission was also proposed. As I listened to the recent Planetary Science Subcommittee meeting, I heard of a fourth proposal for a Discovery Europa mission mentioned in passing, but no details were given.
On one hand, it would seem that the prospects for Discovery missions to the outer planets are poor. The Decadal Survey considered equivalent missions to TiME, JET, and IVO, and estimated their costs at $1.4 to ~$2B, including the launch vehicle. That is well above the approximately $6-700M equivalent budget for Discovery missions. The missions studied by the Survey generally were more capable than the Discovery proposals and in the short time available for the studies were not optimized to fit within a cost target. So can more focused missions designed to cost make up the difference and allow outer planet exploration within a Discovery budget?
NASA's conditions for the current Discovery mission selection may be enabling for outer planet missions. While the principal investigator's (PI) budget for the spacecraft, instruments, operations, and analysis is similar to previous selections ($425M for this selection), NASA now pays for the intermediate class launch vehicle outside of this cost. This is an a significant boost to Discovery mission budgets. (I don't follow launch costs, and so I'm not sure of how much is this adds to the effective budget. $100-200M?) The PI for IVO has said that this change changed the budget from tight under the old rules to doable under the new rules.
In addition to the launch vehicle, for this Discovery selection, NASA will to provide a plutonium ASRG generator to power the craft at no cost to the PI. For missions beyond Jupiter, this would be an enabling technology. For missions to Jupiter, an ASRG may be enabling (solar power is an option there), but could simplify spacecraft design. For example, an Io multi-flyby mission with solar panels would require a scan platform to allow the solar panels to remain sun pointing and the instruments to point toward Io. With an ASRG, the craft can skip this expense and use the entire spacecraft to point the instruments as the Cassini spacecraft does. NASA previously has said that it intends to make ASRGs an option for every other Discovery selection, although I've not heard what they expect to do under the new post Decadal Survey plan to select Discovery missions more frequently (every two years instead of every 3-4 years).
Proposals for outer planet Discovery missions face two hurdles. First, they must be credible and convince the review teams making the selection that they are technically and financially feasible within the Discovery budget. Second, they must be compelling and offer better science than proposals to other destinations. Here, the narrow focus and the long flight times (with operations costs of $7-10M per year, I've heard) may hurt. JET, for example, carries two instruments that would significantly advance our knowledge of Titan and Enceladus. Would a mission to, say, Venus or a comet with five or six instruments provide more compelling science for the dollar?
In the next few weeks, we should learn which three of the 28 proposals (including the outer planet proposals) will become finalists in the current selection for further study to eventually select the winning proposal. If an outer planets mission is among the selections, that would suggest that outer planet Discovery missions are possible. Unless an outer planets mission is eventually selected for flight, however, we won't have a conclusive answer to whether outer planet Discovery missions can clear these two hurdles. The team that has proposed the OSIRIS-REX asteroid sample return New Frontiers mission, originally proposed a similar Discovery mission that was a finalist in a previous selection. The team has reported that while they received top science scores (i.e., the mission was compelling) in the final analysis the mission was judged to be too expensive for the then Discovery mission budget limit.
Editorial Thoughts: I am encouraged by the teams that have proposed outer planet Discovery missions. They include a long list of credible, experienced researchers in planetary exploration who have been around the block a few times. If they are willing to put the time and energy into these proposals, then I feel there's reason for optimism balanced with the observation that outer planet missions inherently carry more costs than equivalent missions to the inner planets. We may know more soon when the current finalists are announced.