Radioisotope power requirements of NASA's candidate Flagship and New Frontiers planetary missions. From the Decadal Survey mission studies.

Based on this table, a reasonable question might be whether the cancellation of the ASRG program will impact the missions that fly. From the list of candidates for Flagship and New Frontiers missions, the answer may be no. There's sufficient Pu-238 for the 2020 rover and Europa Clipper (which could switch to solar power anyway). Under current budget forecasts, a Uranus orbiter wouldn't launch before the new production of Pu-238 becomes available. All the New Frontiers missions on the list could be done with solar power although this would like incur design challenges for the Saturn probe and lunar network.

Where we may see a loss is in the lowest cost class of missions, the Discovery program ($425M to $500M). The expectation had been that NASA would make at least one pair of ASRGs available for a Discovery mission. Engineers and scientists came up with clever ideas for ASRG-based missions – the comet hopper, a Titan lake lander, an orbiter to revisit Titan and Enceladus, a Uranus flyby, and others. With MMRTGs now the only option, NASA needs to hoard its supply of Pu-238. It also has lost the motivation to test a new technology – ASRGs – on a relatively low cost mission. MMRTG technology is already proven.

So NASA's managers made the tough call, and if I were in their shoes, I'd have done the same. I do see two glimmers of hope to resurrect those clever Discovery missions though, or to give engineers the flexibility to use MMRTGs for New Frontiers missions. The first is that Congress for the 2013 budget made it clear that it wanted higher funding for future missions. If this desire becomes policy (and the President's budget office would have to reflect this in their budget projections), then there may be more demand for Pu-238 than NASA is currently envisioning. The second is that if the Europa Clipper design team decides to go with solar power instead of MMRTGs, NASA will have more Pu-238 than missions planned to use it. In this case, they might make MMRTGs available for New Frontiers or even Discovery missions.

You can help build the political support to increase funding for NASA's planetary science program by participating in The Planetary Society's letter writing campaign.

Appendix: Solar Power at Saturn

As part of the 2012 Decadal Survey to create a roadmap for NASA's planetary missions, studies of a number of potential missions were conducted. Two looked at smaller spacecraft to carry and atmospheric probe to Saturn and relay its data back to Earth and to conduct multiple flybys of Enceladus. Both studies concluded that solar cells were possible, but radioisotope power systems were preferable. The following excerpt from the Saturn probe mission study report discusses the challenges of solar power at Saturn.