NASA has decided to stop development of an improved lightweight plutonium power generator, blaming budget constraints and a diminished need for a high-efficiency nuclear power source with the restart of U.S. plutonium production, officials said Sunday.

File photo of a pellet of plutonium-238. Credit: U.S. Department of Energy
 
"We have more plutonium in our science stockpile than was anticipated, such that we don't have the same need," said John Grunsfeld, head of NASA's science mission directorate.

Jim Green, director of NASA's planetary science division, revealed the decision to stop development of Advanced Stirling Radioisotope Generator flight units in a notice to the science community released Friday.

The space agency needs plutonium to fuel power generators on deep space missions that fly too far from the sun to produce electricity with solar panels. NASA's Curiosity rover on Mars, the Cassini spacecraft at Saturn, and the New Horizons probe cruising to a flyby of Pluto are powered by plutonium.

The U.S. government's stockpile of plutonium-238, the special non-weapons grade isotope used to generate electricity for space missions, declined since production of the material stopped in 1988. Forced to ration the leftover plutonium-238, NASA started working on a next-generation nuclear generator that could run on less fuel.

The ASRG is designed to generate more electricity per pound of plutonium-238.

Each ASRG creates between 130 and 140 watts of electricity with 1 kilogram, or about 2.2 pounds, of plutonium-238. More than four times more plutonium would be required to generate the same power in an existing RTG, according to the Energy Department.

Diagram of an Advanced Stirling Radioisotope Generator. Credit: NASA
 
After several studies and a struggle to obtain sufficient funding to restart plutonium-238 production, the U.S. Department of Energy generated tiny quantities of plutonium-238 earlier this year at Oak Ridge National Laboratory in Tennessee.

"The advanced Stirling technology was selected to take advantage of its increased efficiency over the Multi-Mission Radioisotope Thermal Generator (MMRTG), since the supply of plutonium-238 was limited at the time," Green wrote. "Now, with the restart of the Pu-238 production project this year, we expect to have a sufficient supply of Pu-238 for radioisotope power well into the future."

NASA has said the plutonium production rate will be ramped up to about 1.5 kilograms, or 3.3 pounds, yielding enough material to put on a space mission by about 2019.

"We made the decision, based partly on cost and budget, and also on need, to stick with the MMRTGs," Grunsfeld said Sunday. "They need more plutonium, but with the new supply we have that. Then [we can] use our limited budget to fund missions going forward with the MMRTGs."

NASA has directed the Energy Department, which manages the nuclear generator procurement, to end work on the ASRG flight units, according to Green's letter.

NASA science chief John Grunsfeld. Credit: NASA/Bill Ingalls
 
Lockheed Martin Corp. was under contract for development, fabrication and testing of the ASRG ground and flight units. The Stirling convertor was being developed by Sunpower Inc. of Ohio.

The Energy Department does not disclose how much plutonium it has in storage, but the Aerospace Industries Association says the existing stockpile, which was purchased from Russia, will be exhausted after one more large flagship-class nuclear-powered space mission.

NASA plans to launch a copy of the Curiosity Mars rover in 2020, and the robot will likely be powered by an MMRTG. Each MMRTG includes about 8 pounds of plutonium and converts the heat of the isotope's radioactive decay into electricity.

The space agency said ASRG developmental hardware will be transferred to NASA's Glenn Research Center in Ohio for continued research and development.

"For future planetary missions that require radioisotope power systems the flight-proven MMRTG will be made available," Green wrote. "It is important to note that the MMRTG and the ASRG were designed to provide the approximately the same electrical power output."

But the ASRG was lighter and was optimized for smaller spacecraft, such as probes developed under NASA's cost-capped Discovery and New Frontiers programs. Discovery and New Frontiers missions fly more often than flagship-class planetary programs, such as Cassini, Curiosity and the 2020 Mars rover.

An ASRG engineering unit. Credit: NASA
 
Unlike the MMRTG, the Stirling generator design includes moving parts, raising concerns the power device could wear out and limit the life of a spacecraft. Once described by NASA as a "mission-enabling capability," the ASRG consists of highly-efficient Stirling engines coupled with linear alternators.

The MMRTG design uses solid-state thermocouples to convert the plutonium's heat energy into electricity.

"As far as impacts on planetary missions, it means you have to engineer your missions differently," Grunsfeld said. "They may weigh a little bit more, they may have a little bit higher mass, but as far as power levels, we should be able, in principle, to support nearly all the missions that we would with the ASRGs."

NASA made two ASRG flight units available in the last Discovery competition, and two of three finalist missions submitted by scientists relied on the next-generation nuclear technology for electricity. But the agency ultimately selected the solar-powered InSight Mars lander for launch in early 2016.

Officials wanted to complete a flight demonstration of ASRGs on a relatively inexpensive Discovery-class mission before committing the new technology to a more costly flagship probe.

At the time of the InSight selection, NASA said it would made the ASRGs available again in the next Discovery competition, which would choose another planetary mission for launch no earlier than 2020.

Follow Stephen Clark on Twitter: @StephenClark1.