NASA halts work on MMRTG nuclear generator for deep space exploration

MSL’s Pu-238 fueled MMRTG in the laboratory. Credit: NASA

Another blow was dealt to deep space exploration this past weekend.

The announcement comes from Jim Green, NASA's Planetary Science Division Director.

The statement outlines some key changes in NASA's radioisotope program, and will have implications for the future exploration of the outer solar system.

We've written about the impending plutonium shortage and what it means for the future of spaceflight, as well as the recent restart of plutonium production.

NASA is the only space agency that has conducted missions to the outer planets—even the European Space Agency's Huygens lander had to hitch a ride with Cassini to get to Titan—and plutonium made this exploration possible.

Probably the most troubling aspect of the announcement is the discontinuation of procurement by NASA of flight hardware for what was to be NASA's next generation nuclear power-source for exploration, the Advanced Stirling Radioisotope Generator, (ASRG).

This was to replace the Multi-Mission Radioisotope Generator, (MMRTG) that has been in use on spacecraft for decades.

An Advanced Stirling Converter prototype in the laboratory. 

Credit: NASA

The announcement states:

"With an adequate supply of Pu-238… NASA has decided to discontinue procurement of ASRG flight hardware."

"We have given direction to the Department of Energy… to end work on the flight units."

"The hardware procured under this activity will be transferred to the Glenn Research Center to continue development and testing of the Stirling technology."

The announcement cites the current budget-constrained environment that NASA and planetary space exploration finds itself up against.

What the exact future role is of NASA Glenn beyond basic research and development isn't entirely clear, but two ASRG units that were to be flight-ready for missions in 2016 are shelved for now.

The announcement does mention that NASA will continue to utilize flight-proven MMRTG's in the near term, which provide the same approximate power output as the ASRG, albeit with less efficiency.


Plutonium is vital for outer solar system exploration. As you get farther away from the Sun, solar energy ceases to become a viable alternative power source.

Pioneers 10 & 11, Voyagers 1 & 2, Galileo, and Cassini all utilized nuclear RTGs, as does the Mars Curiosity rover and New Horizons mission headed out explore Pluto in during its July 2015 flyby.

NASA's Juno spacecraft scheduled to reach Jupiter in 2016 will be the first-ever mission without an RTG to explore the outer solar system, and it must employ three enormous solar panels to do so.

A labeled cutaway of an ASRG. Credit: 

Wikimedia Commons And although the production of the Pu-238 isotope used in space exploration was a side-benefit of the Cold War, it isn't the same stuff as its isotopic cousin Pu-239, which is used in nuclear weapons. 

Plutonium production in the United States ceased in 1989, and although the U.S. government announced earlier this year that NASA will restart the plutonium production pipeline for space exploration, production levels are only expected to reach 1 to 1.5 kilograms per year.

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