ARLINGTON, Va. — With funds scarce in NASA’s Planetary Science Division, the U.S. space agency’s two active planetary flagship missions find themselves pitted against one another in an upcoming senior review where seasoned scientists will judge which ongoing missions are worthy of more funding.

The next senior review for planetary science missions is scheduled to begin in 2014 and conclude in 2015. Both the rover Curiosity, which landed on Mars last August to begin its two-year primary mission, and the nearly 16-year-old Saturn-system orbiter Cassini are up for review, according to NASA Planetary Science Division director Jim Green.

“Curiosity runs about the same as Cassini, it’s a $50 million to $60 million operation per year,” Green told the Outer Planets Assessment Group during its July 15 meeting in Arlington, Va. “So we have an enormous problem on our hands in terms of trying to accommodate all of our operating missions, and yet move forward with our program.”

Cassini, which launched in 1997 and entered orbit around Saturn in 2004, may not be able to make as strong a case as Curiosity for continued funding. In the last meeting of the Outer Planets Assessment Group in January, Curt Niebur, lead program scientist for NASA’s New Frontiers mission line, said that the spacecraft’s Cassini Plasma Spectrometer instrument, shut down in June 2012 because of electrical problems stemming from suspected tin-whisker growths, was unlikely to be turned back on.

Elsewhere in the Planetary Science Division, budget concerns already have prompted NASA to abandon a concept it had been kicking around since 2012: a ground-based mock-mission known internally as M1 that would have tested a next-generation radioisotope power supply for use in future outer-solar-system expeditions.

“With the budget being what it is, it doesn’t like we’ll be moving forward on that,” Len Dudzinski, program executive for radioisotope power systems at NASA headquarters, said during the Outer Planets Assessment Group meeting.

The planned mock mission was conceived as a way to retire technical risks associated with the Advanced Stirling Radioisotope Generator, or ASRG, that Lockheed Martin Space Systems is building for NASA. At one point, the agency thought it could integrate an ASRG with one of the mission concepts competing with InSight to become the 12th Discovery-class mission. When selection officials chose the solar-powered Mars mission instead, the agency conceived M1, in which an ASRG would have been integrated with a mock spacecraft for long-duration ground testing.

Radioisotope power supplies use the heat from decaying atoms of Plutonium-238 — an isotope not currently produced in the U.S. — to create electricity. NASA is in the middle of an effort, for which it is footing the bill, to resume domestic production of Plutonium-238, of which about 10 usable kilograms remain in the U.S. inventory. The space agency wants to be able to produce around 1.5 kilograms of Plutonium-238 a year, Dudzinski said July 15.

ASRGs will be lighter and more efficient than the current generation of radioisotope power supplies, one of which is now powering Curiosity on Mars. Lockheed Martin Space Systems — working under a $260 million U.S. Department of Energy contract it got in 2008 — is still expected to deliver two flight-ready ASRGs in 2016, Dudzinski said. The goal is to have both the plutonium and the power unit ready to support missions launching in 2020 or later.

“If we don’t get our high-efficiency radioisotope power system … we’re going to be limited in scope with the type of missions NASA as a whole can consider flying for the foreseeable future,” Dudzinski said.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.