NASA is evaluating a new nuclear battery technology based on the radioactive isotope americium-241 that could power spacecraft for up to 433 years, potentially transforming the scope and ambition of deep space exploration by removing the power constraints that have historically limited how long and how far humanity’s robotic emissaries can travel.
The research, conducted in collaboration with the University of Leicester and US national laboratories including Oak Ridge, Idaho, and Los Alamos, represents a significant step beyond the plutonium-238 radioisotope power systems that have powered some of NASA’s most celebrated deep space missions including Voyager, Cassini, and the Curiosity and Perseverance Mars rovers.
The Technology — How a Nuclear Battery Actually Works
Radioisotope power systems generate electricity from a deceptively simple physical process. Radioactive isotopes decay naturally over time, releasing heat as they do. That heat is captured and converted into electricity through free-piston Stirling converters, mechanical devices that transform temperature differences into motion and then into electrical current. The converters have been tested to run continuously for over a decade with minimal wear, making them ideal for the microgravity environments of deep space where maintenance is impossible and reliability is everything.
The system requires no sunlight, no recharging, no external energy input of any kind. It simply sits aboard the spacecraft and converts the steady heat of natural radioactive decay into the electricity that keeps instruments running, communications transmitting, and onboard computers processing data.
Why Americium-241 Over Plutonium-238
The critical difference between the current standard fuel and the new candidate comes down to a single physical property: half-life.
Plutonium-238, which has powered NASA’s deep space missions for decades, has a half-life of approximately 88 years. A half-life is the time it takes for half of a radioactive material to decay, meaning the power output of a plutonium-238 system falls to half its original level after 88 years. For missions like Voyager 1 and Voyager 2, which launched in 1977 and are still operating nearly 50 years later, the declining power output of their radioisotope systems has been a persistent engineering constraint, requiring mission controllers to progressively shut down instruments to conserve the remaining power budget.
Americium-241 has a half-life of approximately 433 years, nearly five times longer than plutonium-238. A spacecraft powered by an americium-241 system launched today would still be operating at more than 85 percent of its original power output when the year 2100 arrives. It would still be generating usable power when the year 2200 arrives. The civilisation that launched it might look very different by the time the battery begins to noticeably decline.
The trade-off is that americium-241 does not produce higher initial power than plutonium-238. For missions that need maximum power output in their early operational years, plutonium-238 remains the stronger option. Americium-241’s advantage is not intensity. It is endurance.
What Missions This Could Enable
The implications of a 433-year power source for space exploration are difficult to fully comprehend in the context of current mission timescales.
The Voyager probes, launched nearly 50 years ago, are now in interstellar space, having crossed the heliopause, the boundary where the Sun’s influence gives way to the interstellar medium. They are the most distant human-made objects in existence. Their plutonium-238 power systems are now so depleted that NASA has had to shut down instrument after instrument to keep the spacecraft operational. Americium-241 systems would have provided those same probes with robust power budgets through the entire journey and well beyond.
Future interstellar precursor missions, probes designed to travel into the outer Solar System and toward nearby stellar neighbourhoods, face the fundamental challenge that they must operate for timescales measured in centuries rather than decades. A nuclear battery that maintains useful power output across that entire timeframe removes one of the central engineering constraints on such missions. The probe can keep its instruments running, keep transmitting data back to Earth, and keep functioning as a scientific platform for as long as its mechanics remain intact rather than as long as its power supply lasts.
Planetary exploration of the outer Solar System, including the ice giant planets Uranus and Neptune, which have each been visited by only a single spacecraft, Voyager 2 in 1986 and 1989 respectively, would benefit from power systems that can sustain orbiter missions for multiple decades without power decline. The proposed missions to these distant planets face years-long transit times and then require sustained orbital operations that current plutonium-238 systems would struggle to maintain across the full mission lifetime.
Where the Technology Stands
Americium-241 is currently in the testing and development phase. It has not replaced plutonium-238 in any operational spacecraft and no mission has been formally approved to use it. The collaboration between NASA, the University of Leicester, and the national laboratories is evaluating the isotope’s practical performance characteristics, the engineering challenges of integrating it into spacecraft power systems, and the supply chain considerations around producing and handling the material at scale.
One practical advantage of americium-241 from a supply perspective is that it occurs as a decay product of plutonium-241, meaning it can be extracted from the reprocessing of spent nuclear fuel. This gives it a different supply chain profile from plutonium-238, which requires dedicated production in nuclear reactors and has historically been produced in limited quantities that have constrained how many deep space missions NASA can power simultaneously.
Early results from the testing programme are described as promising. The path from promising laboratory results to operational spacecraft hardware involves years of engineering development, safety qualification, and mission integration work. But the physical properties that make americium-241 attractive for ultra-long-duration missions are real, measurable, and not subject to further development. The isotope’s 433-year half-life is a fixed property of nuclear physics. The engineering challenge is building a system that can harness it reliably enough to trust a spacecraft to it for a century or more.
The Voyager probes launched in 1977 are still talking to Earth in 2026. With americium-241 power systems, the next generation of deep space explorers could still be talking to whatever Earth looks like in 2426.
This article is based on publicly available research information on americium-241 radioisotope power systems as described in NASA and University of Leicester collaborative research. The technology is in the development and testing phase and has not been deployed in operational spacecraft. This article is for informational purposes only.
