Abstract

Like any machine, spacecraft require electricity to operate. Usually, this power comes from large solar panels. But in “deep space” (beyond Jupiter), the power of sunlight is less than 4% of what reaches Earth. This level is too weak for solar cells to generate the electricity needed by the spacecraft’s scientific instruments designed to investigate the outer solar system’s origin and potential for life. Nuclear fuel is the only primary energy source that will work farther out. For the past 60 years, NASA has employed one form of nuclear energy: radioisotope thermoelectric generators (RTGs), which transform the heat of 5-MeV alpha decay in plutonium-238 into electricity via a low-efficiency (~5%) process called the Seebeck Effect. About 30 missions have used RTGs, each generating a few to a few hundred watts of electrical power, including the two famous Voyager space probes launched in 1977 and the two active Mars rovers.

Pu-238 is rare and expensive ($7,000-$50,000/gram); an RTG fueled with it costs about $1 million per electrical watt. Small amounts (~300 grams/year) of it are produced by neutron bombardment of neptunium-237 in the High Flux Isotope Reactor at ORNL, but this amount is only enough for one big space mission every 15 to 30 years. Many scientifically worthy missions cannot fly because of the lack of Pu-238 fuel. Since the “K” reactors that produced tritium and plutonium-239 for thermonuclear weapons at DOE’s Savannah River National Laboratory were decommissioned at the end of the Cold War, the byproduct Np-237 has not been produced for more than 30 years. All Pu-238 on hand produced by HFIR will be used for the Dragonfly mission in 2028 to Saturn’s moon Titan, after which there will be no more. Last summer the Administration cancelled the RTG program, zeroing it out in FY29. As of this writing, this decision means no more spacecraft will fly beyond Jupiter.

Fortunately, there appears to be a feasible replacement using the cheap ($20/gram) and abundant radioisotope strontium-90 sandwiched with cheap, simple, silicon photovoltaic material. Powerful ~3-MeV beta particles (fast electrons) from Sr-90 are captured by the semiconductor, generating electron-hole pairs that become electrical current in the presence of an applied voltage (hence the name betavoltaic) and at much higher efficiency (~20%) than solar cells. Furthermore, this technique can be scaled downward in size to the tiniest gram-scale spacecraft, which cannot be achieved using RTGs.

In honor of Our Fair City of Oak Ridge, the authors dubbed their innovation “Beta-VOR” (βV), which could rescue this country’s endangered Outer Planets portfolio. We will discuss the advantages of βV, as well as exhibit a conceptual design that we presented to the American Nuclear Society’s Winter Meeting last November in Washington, D.C.

Biographical Sketches

Dr. John E. Gunning, who received his B.S., M.S., and Ph.D. degrees in nuclear engineering from the University of Michigan, has worked for many years in the nuclear field, mostly at the engineering company Bechtel (25 years) and Oak Ridge National Laboratory (10 years). His dissertation “The Structure of Amorphous Arsenic as Determined by Thermal Neutron Scattering” was the first thesis that used data from a pulsed-proton neutron source, called ZING, at Argonne National Laboratory. ZING was the forerunner of the Spallation Neutron Source at ORNL. John moved to Oak Ridge with Bechtel in 1990 and joined ORNL in 2003, retiring from the lab in 2013.

His work experience included gamma-ray imaging related to nuclear medicine research at the University of Michigan Hospital; development of the time-of-flight neutron diffractometer at ZING; nuclear power plant design and licensing, as well as nuclear waste disposal site design and licensing; environmental remediation; restart of Brown’s Ferry Unit 2 Nuclear Power Plant; management of a safety analysis for a Russian facility that stored plutonium removed from Russian nuclear weapons; performance of a quality assurance audit of a Russian nuclear design organization; conversion of Russian plutonium into a mixed oxide for use in light-water reactors; work as a nuclear facility safety engineer for ORNL Building 3019-B; development of a monitor to examine the flow and enrichment of uranium gas at a centrifuge facility, and analysis and evaluation of data from portal monitors at border crossings and ports around the world for detection of the illicit movement of fissile material.

John was chair of the Oak Ridge/Knoxville Section of the American Nuclear Society (ANS), as well as chair and founding chair of the ANS Decontamination and Decommissioning Division and Nuclear Nonproliferation Division, respectively. During retirement he completed America’s Great Loop (6,000 miles by water around the eastern half of the U.S.) with his wife Susan while aboard their Camano 31’ trawler named Nuclear Fishin’. He served as president of Friends of ORNL in 2021-2022.

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Robert G. Kennedy, P.E., callsign K3TVO, is a polyglot systems engineer with 39 years’ experience who does green energy for interesting people all over the world. Like his coauthor, he was president of FORNL (2000-2002) and has a nautical and nuclear background. He is the proud son of Robert Kennedy Jr, third-assistant engineer on NS Savannah, the world’s first nuclear-powered merchant ship, upon which sailed several Oak Ridgers. 

Many moons ago at ORNL, he designed a transplutonium production facility for transmutation of kilogram-quantities of plutonium-240/americium-241/curium-242 into gram-quantities of californium-252 (plus much lesser amounts of einsteinium-254 and fermium-257) for the (never-built, alas) Advanced Neutron Source. A registered professional engineer in California and Tennessee, he spent 1994 in Washington, D.C., working for the U.S. House Subcommittee on Space as the congressional fellow of the American Society of Mechanical Engineers.

He moonlights as co-founder and president of these two grass-roots, community-scale space nonprofits:

• Tennessee Valley Stellar Corporation (www.stellarcorp.tv), which is building a prototype trillionth-scale Cubesat-based sunshade to fly @ SEL₁. Stellarcorp has made an open-source 1-U Cubesat for less than $1000, which will be exhibited. 
• The Institute for Interstellar Studies-US (www.i4is.us), for which I am co-investigator of NASA Innovative Advanced Concepts (NIAC) award-winning grant “Swarming Proxima Centauri” (https://youtu.be/XXW_keR5OIM) featured in The Oak Ridger on October 28 last year ( https://www.oakridger.com/story/news/local/2025/10/27/oak-ridge-engineer-seeks-to-help-make-interstellar-travel-possible/86724495007/ ). In this capacity, he was invited to join NASA’s mission team at Goddard Space Flight Center, where there will be designed an ultralightweight flyby of Neptune’s big moon Triton sometime in the mid-2040s. BetaVOR technology has been baselined for this mission.