LAUREL, Md. — NASA may not have enough plutonium available to power a mission to Uranus recommended by the latest decadal survey until the latter half of the 2030s.
In a May 2 presentation at a meeting of the Outer Planets Assessment Group here, an agency official says NASA is working with the Department of Energy to ensure it has sufficient plutonium-238 for missions projected to launch through the end of the decade. That isotope is used in radioisotope thermoelectric generators (RTGs) to produce electrical power and radioisotope heating units (RHUs) to keep spacecraft warm.
“We are ensuring with the Department of Energy that we can make our mission commitments,” said Len Dudzinski, program executive for radioisotope power systems at NASA Headquarters, at the meeting.
That includes a single Multi-Mission RTG (MMRTG) and up to 24 RHUs for the Dragonfly mission to Saturn’s moon Titan, launching in 2027. NASA is also providing 40 RHUs as part of its contribution to the European Space Agency’s Rosalind Franklin Mars rover, slated for launch in 2028. NASA is reserving two MMRTGs and 20 RHUs for potential use on a New Frontiers mission the agency plans to request proposals for late this year for launch in the early 2030s.
Those missions are supported by increased production of plutonium-238. Dudzinski said NASA and the Department of Energy are more than halfway to a “constant rate production” goal of 1.5 kilograms of the isotope a year, and expect to reach that goal in 2026. “The Department of Energy has said that they can meet our current planning set and current expected launch dates with their current plans for production,” he said.
Those plans, though, do not include Uranus Orbiter and Probe, a mission that was the top-ranked large mission in last year’s planetary science decadal survey. That report recommended NASA start work on the mission as soon as fiscal year 2024 to support a launch in 2031 or 2032, enabling a trajectory that would get the spacecraft to the planet in 13 years.
That schedule is not supported by the current production of plutonium, Dudzinski said. “The decadal survey plan for a 2031 or 2032 launch is, I think, not achievable from the constant rate production plan right now,” he said. The mission, as currently proposed, would require three units of a new Next-Gen RTG design under development by NASA, which each use twice the plutonium of an MMRTG.
It might be possible, he said, to launch the Uranus mission on that schedule if it required only a single Next-Gen RTG and if the selected New Frontiers mission did not require MMRTGs. “That plutonium becomes available to fuel a Next-Gen RTG and launch that early.” A more realistic schedule for having RTGs available for the Uranus mission, he suggested, would be the mid to late 2030s.
While the decadal survey recommended an early 2030s launch of the Uranus Orbiter and Probe, broader funding constraints may keep NASA from pursuing that even if access to RTGs was not an issue. The decadal recommended NASA start work on the mission in fiscal year 2024, but the agency’s 2024 budget request released in March said that the agency now planned to begin formulation studies of the mission in 2025. There are launch opportunities for the mission later in the 2030s, but those require a more circuitous trajectory with travel times of about 15 years.
Other missions being considered may need RTGs, such as Endurance-A, a lunar rover mission to the south polar regions of the moon also endorsed by the decadal survey. Endurance-A would collect samples over a long traverse and deliver them to an Artemis lunar lander mission for return to Earth. Dudzinski said that NASA is still studying both solar power and RTG options for the mission, with the latter requiring a Next-Gen RTG that would otherwise be available for the Uranus mission.
The decadal survey recommended that NASA consider increasing production of plutonium-238 beyond 1.5 kilograms a year “to enable a robust exploration program at the recommended launch cadence.” A report in March by NASA’s Office of Inspector General (OIG) warned of risks of achieving that 1.5-kilogram annual production rate and a lack of “funding flexibility” to increase production above that rate.
“If the demand does increase beyond our current mission planning set, we may need to increase the production throughput at the Department of Energy,” Dudzinski said. The department, he said, is working on a white paper to outline options for doing so, including costs and schedules.
The OIG report also criticized the agency for failure to develop new radioisotope power systems that could offer increased efficiency and thus reduce the amount of plutonium needed. In the last decade NASA has shelved work on two systems, the Advanced Stirling Radioisotope Generator (ASRG) and Enhanced MMRTG, that offered improved efficiency or lifetime over the existing MMRTG.
The agency’s current focus is on the Next-Gen RTG, an updated version of an older design known as GPHS. A prototype called Mod-0 is scheduled to be completed in the fourth quarter of 2024, Dudzinski said. The Mod-1 design, which would be offered for the Uranus, Endurance-A and later missions, will go through a preliminary design review in 2024 with the first Mod-1 ready for fueling in the late 2020s.
The OIG report, though, warned of potential cost and schedule risks to Mod-1 development, including challenges starting up the production line and “a significant risk of parts obsolescence.” He said the agency is working on those issues, particularly with producing thermocouples needed for the RTGs.
There had been plans for a Mod-2 version of Next-Gen RTG with improved thermoelectric technologies for converting the heat produced by the plutonium into electricity. That version is being deemphasized, he said. “The thermoelectric technology for Mod-2 is not deemed mature to be considered for a 2030s availability. We’re focusing on the success of Mod-1.”
NASA had also been working on an alternative technology, called the Dynamic Radioisotope Power System, building on earlier work on ASRG. It could produce as much power as a Next-Gen RTG but use less plutonium than an MMRTG. NASA’s fiscal year 2024 budget request, though, proposes ending work on that project because of funding constraints elsewhere in NASA’s planetary science division.
“The technology has matured a long way since ASRG,” Dudzinski said. “The technology has matured and we are ready to proceed with flight development, but the division can’t afford that.”