Source: Space News | Dan Leone | September 21, 2015
Ever since 2013, when NASA shelved development of a new plutonium battery four times more efficient than those used today, the agency has been forced to come up with creative solutions for missions to sunlight-deprived destinations, or pass on missions that cannot be done with solar power alone.
NASA’s Europa Multiple Flyby mission, which is still in the early Phase A part of its life cycle and not yet under construction, plans to use a large array of solar panels for its journey to the icy Jovian moon. Many believe there could be life on Europa.
Senior people within NASA, including John Grunsfeld, the agency’s associate administrator for science, share that belief — but that was not enough to persuade the agency, which is tightly rationing the plutonium-238 that fuels radioisotope power systems, to spare any of it for the mission.
Where no alternative for a radioisotope power appears possible, NASA has simply taken a pass. Such was the case in 2012 when NASA passed on two nuclear-powered missions and selected the solar-powered Mars lander InSight as its 12th Discovery mission.
The next plutonium-powered mission NASA has committed to is the Mars 2020 rover, a near-copy of the Curiosity rover that landed on the red planet in 2012. The new model, launching in 2020, will be tweaked to drill sample cores of the Martian surface and leave them for a future rover to collect for eventual return to Earth. The power source for Mars 2020 — and for all nuclear-powered NASA missions for the foreseeable future — is the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG).
An MMRTG requires 4 kilograms of plutonium-238 to produce 110 watts of power. The Advanced Stirling Radioisotope Generator, or ASRG, NASA was developing until 2013 would have used only a quarter of the plutonium as the MMRTG to produce the same amount of energy.
The ASRG program is not completely gone. There are about 20 civil servants working on what is left of the program at the Glenn Research Center in Cleveland, NASA spokeswoman Jeannette Owens wrote in an August email.
Glenn has a contract with Sunpower of Athens, Ohio, to build test models of the Advanced Stirling Converters that would power the ASRG.
Sunpower has been working on elements of the ASRG program since 2004, but the company’s three active ASRG-related contracts with Glenn are set to expire this year: one on Sept. 30, the others in December. The combined value of these contracts, awarded in 2010, is roughly $30 million, Owens said.
With ASRGs and their promise of greater efficiency off the table for the near future, the key issue for those planning missions to distant or shadowed solar-system destinations is the restart of U.S. plutonium-238 production.
The U.S. Department of Energy (DoE) will start producing new plutonium-238 for deep-space missions around 2019, but production will ramp up slowly, and NASA still has not committed to setting aside any of the isotope for small missions.
Early next year, the refinery at DoE’s Oak Ridge National Laboratory in Tennessee will restart for the first time in 27 years to produce a test-batch of the isotope, a DoE official told the NASA-chartered Outer Planets Assessment Group (OPAG) during its August meeting in Laurel, Maryland.
If the sample tests cleanly, Oak Ridge will start pumping out bigger batches, beginning with 400 grams in 2019, Rebecca Onuschak, program director for infrastructure capabilities in DoE’s Office of Space and Defense Power Systems, said in a presentation to OPAG.
That amount is less than a third of the annual output NASA and the Energy Department were shooting for after Congress transferred the budget for critical parts of DoE’s plutonium-handling infrastructure to the space agency in 2012.
“We’ll gradually ramp up to the 1.5-kilogram rate as the funding becomes available,” Onuschak said.
Based on current budget projections, DoE could produce 1.5 kilograms a year by the mid-2020s, Onuschak told OPAG.
From 2012 through 2015, NASA will have spent just over $165 million on DoE’s plutonium-handling infrastructure, and about $51.5 million on the production equipment and personnel, according to a NASA slide presented to OPAG.
DoE refines plutonium-238 at Oak Ridge. The fuel is pressed into pellets that can fit into MMRTGs at the department’s Los Alamos National Laboratory in Los Alamos, New Mexico. Los Alamos will test the sample batch of plutonium-238 DoE hopes to produce next year, Onuschak said.
There are still 35 kilograms of usable plutonium-238 left in the U.S. stockpile for civil space missions — additional plutonium is reserved for the military — but only 17 kilograms of the material meet DoE’s minimum energy requirements for space.
NASA’s Mars 2020 rover will get about 4 kilograms. After that, DoE could fuel another three MMRTGs, although it would have to rely on some of the below-specification plutonium-238. The out-of-spec material would reduce mission life, but not make a mission unfeasible, Onuschak said.
If new plutonium-238 begins showing up in 2019, it can be blended with existing stock to replenish the old material’s energy content. That would allow DoE to fuel additional nuclear batteries, although Onuschak could not say exactly how many.
Meanwhile, NASA’s director of planetary science, Jim Green, dropped hints during the OPAG meeting about which post-Mars 2020 missions stand a chance of getting some plutonium-238.
Those proposing medium-sized, principal investigator-led missions under NASA’s fourth New Frontiers competition, which will begin some time after Sept. 30, likely will get access to radioisotope power supplies, Green said. NASA also wants to reserve a cache of plutonium-238 for a possible mission to Uranus or Neptune in the late 2020s or 2030s, he said.
Small missions, however, maybe be out of luck, at least in the near-term.
When an OPAG audience member asked if NASA will provide a nuclear battery for the 14th mission in the agency’s Discovery line of competitively awarded small solar-system probes, Green said only that he would “consider” it.
For its 13th Discovery mission, competition for which is underway, NASA banned radioisotope power supplies. The winner of the competition, which NASA will select in 2016, will launch in 2021.