In 1977, two spacecraft, Voyager 1 and Voyager 2, launched toward the unknown. Nearly fifty years later, they remain alive at the edge of interstellar space, still whispering faint signals back to Earth. That longevity is not due to solar panels or batteries, which would have failed decades ago. Instead, it comes from a small nuclear power source. Plutonium-238 is packed inside radioisotope thermoelectric generators, or RTGs.

Now, scientists believe a new fuel, Americium, may reshape the future of deep space exploration. It could power missions not for decades, but for centuries. It could give countries independence from limited plutonium supplies. And it could enable new types of spacecraft that drift between stars long after governments and civilizations have changed. Understanding how this future becomes possible begins with a simple question. What makes nuclear batteries so special?

Why do spacecraft need nuclear power?

Solar panels are excellent near Earth, where sunlight is abundant. They power satellites, the International Space Station, and even some Mars rovers. But sunlight fades quickly as distance grows. Jupiter receives twenty-five times less sunlight than Earth. At Pluto, the light is a thousand times weaker.

If the Voyager spacecraft had relied on solar power, each of them would have needed solar arrays larger than a football field. Instead, each carried an RTG about the size of a trash can. An RTG works by converting the heat from radioactive decay into electricity. Inside the device is a fuel pellet, typically made of plutonium-238. As plutonium atoms decay, they release energy in the form of heat. Thermocouples convert that heat into electricity. There are no moving parts and no mechanisms that can fail easily. The result is steady, reliable power that lasts for decades.

Voyager’s three RTGs held nearly five kilograms of plutonium-238 each. At its launch in 1977, it produced approximately 470 watts of power. Almost half a century later, they still deliver over 200 watts of power. Enough to keep vital instruments alive, even as engineers shut down systems to extend the mission. Plutonium-238 is not weapons plutonium. It cannot undergo runaway fission, cannot go critical, and cannot melt down. It simply decays in a controlled, steady way, releasing heat.

How Plutonium-238 became the gold standard

Plutonium-238 has a half-life of 88 years, a balance between energy output and longevity. It was first used in space in 1969. Early systems powered satellites and scientific stations left on the Moon during the Apollo missions. As missions grew more ambitious, RTGs traveled to Jupiter, Saturn, Pluto, and beyond.

Cassini, which orbited Saturn from 2004 to 2017, carried 33 kilograms of plutonium-238. Without that power source, it could not have survived thirteen years in Saturn’s cold, distant darkness. The problem is not performance, it’s supply. Plutonium-238 does not occur naturally. It must be manufactured in nuclear reactors. During the Cold War, the US and the Soviet Union produced significant amounts. After the Cold War ended, the US stockpile of nuclear weapons slowly diminished.

By the 2000s, NASA faced serious shortages. Curiosity’s RTG in 2012 relied on some of the last major reserves. Restarting production took years, and it was not until 2015 that new plutonium-238 was produced again at Oak Ridge National Laboratory. Even now, annual production amounts to only a few hundred grams.

A single deep space mission might require five kilograms or more. At current rates, NASA cannot sustain a growing set of missions using plutonium alone. That challenge has opened the door to a surprising alternative.

Americium: A long-life fuel hidden in nuclear waste

Americium is one of the lesser-known synthetic elements, first created in 1944 during the Manhattan Project. The isotope of interest for space is americium-241. Its half-life is a staggering 432 years, five times longer than plutonium-238.

Such longevity makes Americium attractive for missions designed to last centuries, not decades. But the most compelling advantage is its supply. Americium-241 forms naturally inside nuclear waste as plutonium-241 decays. It accumulates over time.

In the UK, large stocks of civil nuclear waste contain significant quantities of americium-241. That makes the fuel not only long-lasting but also readily accessible. Instead of building new reactors to produce plutonium, agencies can extract Americium from existing waste, a form of recycling at a planetary scale.

Americium offers sustainability, availability, and strategic independence. For Europe in particular, it provides a path toward deep-space power systems that do not rely on the United States for plutonium.

Comparing Plutonium and Americium

Plutonium-238 remains the preferred isotope for missions requiring high power. It releases more heat per gram, is chemically stable, and yields about 0.5 watts of thermal power per gram. Americium-241 produces only about 0.1 watts per gram, five times less. To generate the same power output, an americium RTG must be significantly larger or heavier. That is a challenge in spaceflight, where mass is a precious resource.

Plutonium is the high-performance option. Compact, hot, and ideal for missions like Perseverance or Curiosity, which must power drills, cameras, lasers, and communications systems. Americium is the endurance option. Cooler, bulkier, and ideal for small, low-power probes intended to survive for centuries.

The missions of Americium could make it possible

Europe is already pursuing americium-based RTGs. For more than a decade, the University of Leicester has been developing americium power systems in partnership with the European Space Agency and the UK Space Agency. Their work spans full-sized RTGs and small radioisotope heater units designed to keep instruments warm on frigid worlds.

Americium is especially attractive for long, slow, or distant missions with minimal power demands, such as probes studying geological processes on icy moons or deep-space instruments designed to drift through interstellar space for hundreds of years.

NASA’s proposed Interstellar Probe, a mission concept that would travel 150 billion kilometers from Earth, would require a power source lasting not decades but generations. This is precisely where Americium excels. Americium RTGs also provide independence. When Europe lost access to Russian power systems due to geopolitical tensions, Americium became a key strategy for powering future missions, such as the Rosalind Franklin Mars rover.

Human missions to Mars would likely use a combination of power systems. Americium could play a supporting role, providing steady heat and electricity over multi-year journeys.

The challenge of power density and the promise of Stirling engines

Americium’s biggest drawback is its low heat output. Larger power systems result in heavier launch masses. To overcome this, researchers are revisiting a centuries-old technology—the Stirling engine. A Stirling converter uses a closed-cycle system where a working fluid expands and contracts, driving a piston connected to an alternator. Unlike an open-cycle car engine, the system retains its working fluid completely, making it suitable for space applications.

Traditional RTGs utilize thermoelectrics, which are reliable but inefficient, often achieving only five percent efficiency. Stirling engines can convert heat to electricity with an efficiency of 25 percent or more. That means either more electrical power from the same amount of fuel, or the same power with significantly less fuel.

Stirling engines introduce moving parts, which also raises reliability concerns in space. However, Americium’s steady heat output enables RTG designs with multiple Stirling converters operating in tandem. If one fails, the others compensate, preserving power output. Testing continues, and americium-based Stirling RTGs have not yet flown. But progress is promising, and this combination of long-lived fuel and efficient energy conversion could transform deep-space power systems.

A new era of power independence

For decades, the United States’ control over plutonium-238 production gave NASA a near-monopoly on deep space missions. Other nations, including those in Europe, often had to rely on solar power or accept limitations in mission design.

Americium RTGs change that. They create a new supply chain, allowing nations to develop their own power systems independently. If americium systems mature, both fuels may coexist—plutonium for high-power missions, Americium for long-duration or low-power ones.

Americium may also find uses on Earth. Power systems based on Americium could support remote military operations, deep-sea exploration, or any environment where steady power is needed far from civilization.

The future powered by Americium

Plutonium-238 made possible the great journeys of the 20th and early 21st centuries, from the Moon to Saturn to the edge of interstellar space. However, as ambitions grow and plutonium supplies become scarce, a new fuel may shape the next era of exploration.

Americium does not burn as brightly, but it burns far longer. It offers endurance over raw power, longevity over intensity. And in deep space, where distances stretch across centuries, endurance may be the quality that matters most.

The same element that sits quietly in household smoke detectors could one day power instruments drifting between stars, or probes exploring alien oceans in the dark. The race to explore the cosmos may depend not only on rockets and telescopes, but on who controls the nuclear materials that keep spacecraft alive long after launch.