NASA’s decision to discontinue work on an advanced nuclear battery for deep-space exploration was perfectly reasonable and defensible under current circumstances, but it was very unfortunate just the same. It means, essentially, that NASA lacks a compelling reason to advance the state of the art in a technology that is key to robotic exploration of the far corners of the solar system. 

In mid-November, NASA quietly informed members of the planetary science community that it was pulling the plug on development of the Advanced Stirling Radioisotope Generator (ASRG), a more efficient version of the radioisotope power sources that have long provided electricity to probes operating too far from the sun to rely on solar arrays. NASA had hoped to demonstrate the technology, years in development, on a space mission in 2016.

The reasons for canceling the project were straightforward enough. With the U.S. Department of Energy having restarted production of previously scarce plutonium-238, the fuel source for nuclear batteries, there is less of a need to use it four times more efficiently, as the ASRG would have done. Moreover, the cash-strapped U.S. space agency has significantly curtailed its ambitions for deep-space exploration, deferring until further notice plans for missions that would return samples from Mars or probe mysteries of the jovian moon Europa.

NASA was spending roughly $50 million per year on the Lockheed Martin-led ASRG development effort, and estimated that $170 million more would be needed to have the first flight model ready for a possible 2016 demonstration. But having chosen a solar-powered Mars probe for its 2016 Discovery mission flight opportunity, the agency, which is under pressure to scrimp and save everywhere it can, elected to shelve the project.

While perfectly logical, the move serves as yet another reminder of an unfortunate reality: NASA’s planetary science program doesn’t have any mission in the works that might require a fourfold increase in the efficiency of nuclear power sources. Moreover, even if an ambitious new deep-space probe were to somehow get past the conceptual stage at NASA, it likely would incorporate a proven power plant — the ASRG, unlike traditional plutonium-powered space batteries, has moving parts — given the agency’s desire to minimize risk on flagship-class missions. That means that any such mission, all else being equal, will have less capability than otherwise would be the case. 

The demise of the ASRG once again demonstrates the inherent vulnerability of programs to develop a technology that is not designated to fly on an approved mission. One has to wonder whether NASA effectively sealed the ASRG’s fate when it did not select the Titan Mare Explorer or the Comet Hopper, both of which would have relied on the new generator, as the next mission in its long-running Discovery-series of low-cost planetary missions. It’s also reasonable to ask whether NASA, by shelving the ASRG, has also closed the door on any number of future discoveries in deep space.