Plutonium-238 pellet
Plutonium-238 pellet. Credit: U.S. Department of Energy

WASHINGTON — Ohio’s U.S. Senate delegation ordered up an extensive report on the federal infrastructure required to produce both the nuclear batteries that power NASA missions to dark and distant corners of the solar system, and the plutonium isotope that fuels those batteries.

Under the Efficient Space Exploration Act, filed July 22 by U.S. Sens. Sherrod Brown (D-Ohio) and Robert Portman (R-Ohio), NASA and the White House Office of Science and Technology Policy would take the lead on a study to determine the space agency’s exact requirements for radioisotope power systems, the plutonium-238 that fuels them, and the risks to planned missions if those needs are not met.

The bill — filed at a time when the U.S. plutonium-238 supply is dwindling and budget cuts forced NASA to cancel development of a more efficient nuclear battery under development at a NASA facility in Ohio — also directs the White House to ensure the Department of Energy, which is responsible for U.S. plutonium production, does not overcharge NASA for plutonium infrastructure upgrades at DOE’s Los Alamos National Laboratory in New Mexico.

NASA’s New Horizons probe, which flew past Pluto July 14, is powered by plutonium-238. (NASA artist’s concept)
NASA’s New Horizons probe, which flew past Pluto July 14, is powered by plutonium-238. (NASA artist’s concept)

NASA is required by law to fund DOE’s plutonium-238 production because Congress in 2012 deemed the space agency the primary user of the fuel. The report Brown and Portman want would be due to Congress no later than 180 days after the bill is signed. First and foremost, according to the bill, the report should “detail the current projected mission requirements and associated time frames for radioisotope power systems and radioisotope power system material.”

That requirement was noticeably absent from a 135-page Nuclear Power Assessment report completed by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, in November but not released until June 1 because of nuclear security concerns.

Prepared for June Zakrajsek, radioisotope power systems programming planning and assessment manager at NASA’s Glenn Research Center in Cleveland, the Nuclear Power Assessment was “not intended to represent NASA policy or planning.” It did, however, incorporate publicly available data about planned NASA missions into a broad summary of the agency’s nuclear power needs.

Planetary probes and landers operating where sunlight is scarce, either due to distance from the sun or shadows, trade solar panels for nuclear batteries known as radioisotope power systems. These units convert waste heat from the radioactive decay of small plutonium-238 pellets into electricity.

The Ohio delegation’s interest in nuclear-powered space missions is two-fold. NASA’s Advanced Stirling Radioisotope Generator (ASRG) program, or what is left of it, is based at Glenn. Also, Sunpower of Athens, Ohio ­— a specialty manufacturing company acquired in 2013 by Paoli, Pennsylvania-based Ametek ­— is providing Glenn with test models of the Advanced Stirling Converters that would power the ASRG.

Sunpower has been doing technology development work on the converters since 2004 under contract to Glenn, according to slides the company briefed June 29 at the Stirling Technical Interchange Meeting at the Ohio Aerospace Institute in Cleveland. The latest Glenn contract calls for the company to produce eight engineering units.

Sunpower was also supposed to build flight converters for a space-worthy ASRG as a subcontractor to former ASRG prime Lockheed Martin Space Systems. However, NASA canceled substantially all ASRG development 2013, along with the roughly $300 million NASA-funded and DOE-managed ASRG prime contract Lockheed had held since 2008.


When he pulled the plug on ASRG in 2013, Jim Green, NASA’s planetary science director, said canceling that program spared the agency from cutting others, such as the competitively awarded Discovery and New Frontiers lines of small- and medium-sized solar system missions.

From 2008 to 2013, NASA pumped about $270 million into ASRG work, most of which went to Lockheed. The money bought the space agency a partially completed ASRG qualification unit.

NASA spokesman Joshua Buck did not immediately reply to a request for comment about the exact number and value of Sunpower’s various Advanced Stirling Converter contracts. Jeffrey Hatfield, a Sunpower vice president and site manager quoted in a joint Brown-Portman press release about the new space nuclear power bill, did not return a call July 28.

Stirling motors, which date back to the industrial revolution and use an external heat source to drive a piston, are more efficient than the solid-state thermoelectric converters used in the current generation nuclear battery, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). An ASRG could produce about 140 watts of power using slightly less than a kilogram of plutonium-238 — about a fourth of the fuel the heavier MMRTG needs for the same output.

The Curiosity rover that landed on Mars in 2012 uses a single MMRTG, as will the Mars 2020 sample-collecting rover NASA plans to send to the red planet that year. Mars 2020 will be based heavily on Curiosity’s design and has first dibs on whatever plutonium-238 it needs.

As for future missions, there is enough plutonium-238 in the U.S. stockpile for two more MMRTGs, now that managers of NASA’s Europa Clipper mission, slated for launch in the mid 2020s, have decided to use solar power.

In February, Alice Caponiti, DOE’s director of space and defense power systems, said once NASA-funded repairs to DOE’s Los Alamos facility are completed, the Energy Department will start producing 1.5 kilograms of plutonium-238 a year to supplement a civil-space stockpile of about 35 kilograms.

Only 17 kilograms of the civil stockpile — additional plutonium-238 is reserved for the military — meet DOE’s minimum energy requirements for space missions, according to Caponiti.

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.