European scientists are developing a breed of battery for space missions that is powered by nuclear waste. The European Space Agency (ESA) hopes that the technology will, by the end of the decade, allow it to operate spacecraft that don’t rely on solar panels and can explore the Moon and far-off reaches of the Solar System without relying on equipment from international partners.
Ministers at ESA’s ministerial council meeting in Paris on 22 and 23 November agreed to fund a €29-million (US$30-million) programme called European Devices Using Radioisotope Energy (ENDURE). This aims to develop long-lasting heat and electricity units powered by the radioactive element americium-241, in time for a series of ESA Moon 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.
Americium, a by-product of plutonium decay, has never been used as a fuel. For missions in which solar power would not suffice — either because of shade or because of distance from the Sun — ESA has relied on US or Russian partners, which have used plutonium-238 batteries to power missions since the space race. NASA built plutonium batteries, for example, that warmed 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.
The lack of a power source has long restricted the solo missions that European scientists propose, and limited others. The agency felt its lack of radioisotope power keenly in 2014, when its comet-landing Philae probe was operational for less than three days because it ended up in a shaded spot 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.
Better than plutonium?
Americium’s big advantage over plutonium is that it is cheaper and more abundant, repurposing waste that would otherwise be useless, says Véronique Ferlet-Cavrois, who co-leads the ENDURE initiative at ESTEC.
Plutonium-238 is made in a two-stage process that involves irradiating a neptunium target with neutrons. Researchers at the UK government’s National Nuclear Laboratory (NNL) in Sellafield have shown that americium can be extracted from reprocessed nuclear fuel used in civil power plants and made into fuel pellets, which form the core of the batteries. Part of the ENDURE programme will include raising americium production capacity to what is needed for batteries, says Hatton.
Americium has a longer half-life than plutonium-238, which means it lasts longer but packs less power per gram. But because americium is more readily available, producing one watt of power costs about one-fifth as much as it does using plutonium, says Markus Landgraf, who coordinates work on future lunar missions at ESTEC.
Over the next three years, the ENDURE team will develop prototypes into models that can be tested in mission-like conditions, as precursors to usable devices. In a collaboration with NNL, a team at the University of Leicester, UK, has developed two types of device: a radioisotope heating unit, which warms instruments with heat produced in the decaying americium, and radioisotope thermoelectric generators (RTGs), which use the heat to produce electricity by creating a temperature difference across metal plates.
The researchers designed both types of device to account for americium’s higher volume for a given power output, and cooler temperatures, compared with plutonium, says Richard Ambrosi, a physicist and specialist in space power systems, who leads the team at the University of Leicester.
Safety is also crucial, because of the use of radioactive materials. The units are encapsulated in layers including a platinum alloy, which seal in the americium while allowing heat to escape, he says. The programme’s next phase will focus on safety testing, so that the americium units can be certified for launch. Tests will include monitoring the behaviour of components at high temperatures and under impact — for example, during an explosion on the launch pad — to ensure that radioactive material would not leak. “We have to be able to survive a significant set of very extreme scenarios,” says Ambrosi.
Batteries on the Moon
Once developed, the same basic power system could be reused on any missions for which solar energy is unavailable, says Ferlet-Cavrois. This is the case during nights on the Moon, which last 14 Earth days, and on expeditions to the Solar System beyond Jupiter. To survive the harsh lunar night, China’s active Moon rover, Chang’e-4, uses plutonium heating units built in collaboration with Russia.
ESA’s first target for launching americium power sources is its Argonaut Moon lander, scheduled to launch in the early 2030s. The Argonaut missions would conduct long studies on the lunar surface and support astronauts working there, says Landgraf. And in the 2040s, ESA hopes to power a mission to the ice giants Uranus and Neptune, says Ferlet-Cavrois. These planets have been studied only during fly-bys by NASA’s Voyager 2 probe in the 1980s.
Americium’s availability, and the challenges of producing plutonium-238, mean that NASA might want to use it too, says Landgraf. The agency is assessing its ability to produce enough RTGs for its coming missions. For its Artemis programme, which aims to establish a long-term presence on the Moon, “they consider our americium very interesting”, he says.
It has taken more than a decade of research to get the americium technology to the stage at which it can be developed for real missions, says Ambrosi. “The excitement is actually quite palpable at the moment. We’ve been working on this for a long time,” he says.