KENNEDY SPACE CENTER, Fla. — NASA has stopped production of a next-generation radioisotope power system that uses less plutonium, a decision that could also stop a congressional effort to revive a pair of planetary science missions.

In a statement emailed to the planetary science community late the afternoon of Nov. 15, Jim Green, director of NASA’s Planetary Science Division, said the agency was halting funding of the development of the Advanced Sterling Radioisotope Generator (ASRG).

“With an adequate supply of Pu-238 [plutonium-238], and considering the current budget-constrained environment, NASA has decided to discontinue procurement of ASRG flight hardware,” Green said, adding that NASA had directed the U.S. Department of Energy, which was managing the ASRG work, to stop production.

The ASRG has been touted by NASA as a solution to the limited supply of plutonium-238 available for future missions. Like NASA’s existing Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), the ASRG converts heat from the radioactive decay of plutonium into electricity. However, the ASRG is designed to be much more efficient than the MMRTG, producing the same amount of electrical power with less than 25 percent of the plutonium.

John Grunsfeld, NASA associate administrator for science, said the ongoing efforts to restart plutonium-238 production have eliminated the urgency to develop the ASRG.

“We have more plutonium in our science stockpile than we anticipated, such that we don’t have the same need,” he said at a press conference here one day ahead of the Nov. 18 launch of the Mars-bound Maven orbiter. “So we made the decision, based partly on cost and budget, and also on need, to stick with the MMRTGs. They need more plutonium, but with the new supply we have that.”

In an interview here Nov. 17, Green said that the ASRG program currently costs about $50 million per year, and stopping production now would cumulatively save $170 million through 2016, when the first two ASRG flight units were to be completed. NASA’s Glenn Research Center in Cleveland will continue ASRG technology development work, he said, but will not produce flight units. Lockheed Martin Space Systems was the lead contractor for ASRG.

Another factor in the decision was the lack of users for the ASRG. The 2016 date for the production of the first ASRG unit would have allowed their use on NASA’s next Discovery-class planetary science mission. Two of the three finalists for that mission, Comet Hopper (CHopper) and Titan Mare Explorer (TiME), proposed using ASRGs. However, in August 2012, NASA selected InSight, a solar-powered Mars lander. An ongoing study by the Jet Propulsion Laboratory of the proposed Europa Clipper mission has also rejected the use of ASRGs, opting for conventional MMRTGs or large solar arrays.

The decision to end ASRG production could jeopardize a congressional effort to revive CHopper and TiME. The report accompanying the Senate version of the 2014 appropriations bill that would fund NASA directs NASA to initiate studies on an additional Discovery mission from the most recent competition “with the highest scientific value that meets the program’s cost cap.” Without an available ASRG, CHopper and TiME would likely be excluded.

The decision to end ASRG production has raised concern among some planetary scientists, who believed the ASRG would enable smaller and less-expensive outer planets missions in the future. In addition to requiring less plutonium, the ASRG is lighter and produces less waste heat than the MMRTG.

Grunsfeld said missions to the outer solar system are still viable without the ASRG. “It means you have to engineer your missions differently, and they may weigh a little bit more,” he said. “We should be able, in principle, to support nearly all of the missions we would have with the ASRGs.”