European scientists are developing a type of battery for space missions that runs on nuclear waste. The European Space Agency (ESA) hopes that by the end of the decade, this technology will make it possible to operate spacecraft that do not rely on solar cells and can explore the moon and distant parts of the solar system without relying on equipment from international partners.

European Space Agency to fund long-lasting power for Moon missions

At the ESA Council of Ministers meeting Nov. 22-23 in Paris, ministers agreed to fund a €29 million ($30 million) program called European Devices Using Radioisotope Energy (ENDURE). The goal is to develop long-lived heat and power units fueled by the radioactive element americium-241 in time for a series of ESA lunar missions in the early 2030s.

“If we want to have autonomy in exploration, we need these capabilities,” says Jason Hatton, a co-leader of ENDURE, based at the European Space Research and Technology Center (ESTEC) in Noordwijk, the Netherlands. ESA’s growing space ambitions mean it needs its own source of long-lasting power, says Hatton.

Europe to use plutonium-238 to power its far-flung probes

Americium, a byproduct of plutonium decay, has never been used as fuel. For missions where solar power is insufficient - either because of shadow or distance from the sun - ESA has relied on U.S. or Russian partners who have used plutonium-238 batteries to power missions since the space race. NASA, for example, built plutonium batteries that heated the Huygens probe during its descent to Saturn's moon Titan in 2005. But plutonium-238 has been in short supply over the past decade and is expensive to produce.

And ESA severed ties with Russia after the country invaded Ukraine. “The current political situation demonstrates that you cannot always rely on partners”, says Athena Coustenis, an astrophysicist at the Paris Observatory in Meudon, France, who chairs an ESA advisory committee that backed the new programme.

U.S. scientist sees hope of exploring ice giants through solar power

The lack of a power source has long hampered solo missions proposed by European scientists and limited others. The agency felt the lack of radioisotope power keenly in 2014, when its Philae probe, which landed on the comet, was operational for less than three days because it had landed in a shaded area where its solar panels were useless. “For years, European scientists have been saying that if you want to go far, or to dark and cold places, there is no other way,” says Coustenis.

Americium - it's easy to make and cheaper than plutonium for lunar batteries

The big advantage of americium over plutonium is that it is cheaper and more abundant, so waste that would otherwise be useless can be reused, explains Véronique Ferlet-Cavrois, who co-leads the ENDURE initiative at ESTEC.

Plutonium-238 is produced in a two-step process that involves irradiating a neptunium target with neutrons. Researchers at the U.K. government's National Nuclear Laboratory (NNL) in Sellafield have shown that americium can be extracted from reprocessed nuclear fuel used in civilian power plants and processed into fuel pellets that form the core of batteries. Part of the ENDURE program will involve increasing americium production capacity to the level needed for batteries, Hatton said.

Americium has a longer half-life than plutonium-238, which means it lasts longer but provides less energy per gram. However, because americium is more readily available, generating one watt of power costs only about one-fifth as much as using plutonium, says Markus Landgraf, who coordinates work on future lunar missions at ESTEC.

A plutonium base on the Moon could power sensitive instruments like radon - researchers

Over the next three years, the ENDURE team will develop prototypes into models that can be tested under mission-like conditions and serve as precursors to mission-ready devices. In collaboration with NNL, a team at the University of Leicester in the United Kingdom has developed two types of devices: a radioisotope heating unit that heats instruments with heat generated in decaying americium, and radioisotope thermoelectric generators (RTGs) that use the heat to generate electricity by creating a temperature difference between metal plates.

Researchers developed both types of devices to account for americium's higher volume for a given power output and cooler temperatures compared to plutonium, says Richard Ambrosi, a physicist and space power systems specialist who leads the team at the University of Leicester.

Safety is also critical because of the use of radioactive materials. The units are encapsulated in platinum alloy layers that trap americium while allowing heat to escape, he says. The next phase of the program will focus on safety testing so the americium units can be certified for launch. Testing will include monitoring the behavior of the components at high temperatures and during impacts - such as an explosion on the launch pad - to ensure that no radioactive material escapes. "We have to be able to withstand a whole range of very extreme scenarios," Ambrosi says.

Batteries powered by plutonium-238 could be reused on lunar missions

Once developed, the same basic power system could be reused on all missions where solar power is not available, Ferlet-Cavrois said. This is the case for nights on the moon that last 14 Earth days and expeditions to the solar system beyond Jupiter. To survive the harsh lunar night, China's Chang'e-4 active lunar rover uses plutonium heating units built in collaboration with Russia.

ESA's first target for the use of americium power sources is the Argonaut lunar lander, scheduled for launch in the early 2030s. The Argonaut missions would conduct long studies on the lunar surface and support astronauts working there, Landgraf says. And in the 2040s, ESA hopes to launch a mission to the ice giants Uranus and Neptune, Ferlet-Cavrois says. Those planets have only been studied during flybys of NASA's Voyager 2 spacecraft in the 1980s.

The availability of americium and the difficulties in producing plutonium-238 mean NASA may want to use it as well, Landgraf says. The agency is currently evaluating its ability to produce enough RTGs for its upcoming missions. For its Artemis program, which aims for a long-term presence on the moon, "they think our americium is very interesting," he says.

According to Ambrosi, it has taken more than a decade of research to get americium technology to the point where it can be developed for real missions. "The excitement is pretty palpable right now. We've been working on this for a long time," he says.