European scientists are developing a variety of batteries powered by nuclear waste for space missions.  By the end of the century, the European Space Agency (ESA) hopes the technology will allow it to operate spacecraft that do not rely on solar panels and be able to explore the moon and distant solar system without relying on equipment from international partners.  

At a meeting of ESA's Council of ministers in Paris on November 22nd and 23rd, ministers agreed to provide €29m ($30m) for a scheme called the European Radioisotope Energy Facility (ENDURE).  The program aims to develop a long-term thermal and electrical installation powered by the radioactive element americium-241 in time to support a series of ESA lunar missions in the early 2030s.  

"If we want to take ownership of our exploration, we need these capabilities," says Jason Hatton, co-leader of the ENDURE project at the European Space Research and Technology Centre (ESTEC) in Nordwijk, the Netherlands.  Hatton said ESA's growing space ambitions meant it needed its own enduring source of power.  

Element 241  

Americium is a byproduct of the decay of plutonium and has never been used as fuel.  For missions where solar isn't enough -- whether because of shade 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, made plutonium batteries that kept the Huygens probe warm during its descent to Saturn's moon Titan in 2005.  But plutonium-238 has been in short supply for the past decade and is expensive to produce.  

Moreover, after the conflict between Russia and Ukraine, ESA broke off relations with the country.  "The current political situation shows that you can't always rely on partners," says Athena Coustenis, an astrophysicist at the Paris Observatory in Maudon, France.  He is chairman of the ESA advisory committee that supports the new plan.  

The lack of a power source has long limited the individual missions proposed by European scientists, as well as others.  The agency felt its lack of radioisotope power acutely in 2014, when its comet-landing Philae probe operated for less than three days because it ended up in a shady place where its solar panels were useless.  "For years, European scientists have been saying that if you want to go further, or go to dark and cold places, there is no other way," Coustenis said.  

Better than plutonium?  

"Americium's great advantage over plutonium is that it is cheaper, more abundant and can be reused as otherwise useless waste," said Veronique Ferlet-Cavrois, co-leader of ESTEC's ENDURE program.  

Plutonium-238 is made in a two-stage process that involves irradiating neptunium targets with neutrons.  Researchers at the UK Government's National Nuclear Laboratory (NNL) at Sellafield have shown that americium can be extracted from reprocessed nuclear fuel used in civilian power plants and made into fuel particles that form the core of batteries.  Part of the ENDURE plan will include increasing americium capacity to meet battery needs, Hatton said.  

Americium has a longer half-life than plutonium-238, which means it has a longer life but less power per gram.  But because americium is more readily available, the cost of producing a watt of power is about one-fifth that of 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 under similar mission conditions, as a prelude to usable devices.  In collaboration with NNL, a team at the University of Leicester in the United Kingdom developed two types of devices: radioisotope heaters, which use heat from decaying americium to heat instruments;  And radioisotope thermoelectric generators (RTGS), which use heat to generate electricity by creating temperature differences in metal plates.  

Richard Ambrosi, a physicist and space power systems expert at the University of Leicester, said the researchers designed both types of equipment to take into account americium's larger volume and lower temperature than plutonium for a given power output.  

Safety is also crucial because of the use of radioactive materials.  He stated that these units are encapsulated in layers, including platinum alloys, which seal the americium while allowing heat to escape.  The next phase of the program will focus on safety testing so that the americium device can be certified for launch.  The tests will include monitoring how the components behave under heat and shock - for example, in the event of an explosion on the launch pad - to ensure radioactive material does not leak out.  "We have to be able to survive a very extreme set of circumstances," Ambrosi said.  

Batteries on the moon  

"Once developed, the same basic power system can be reused for any mission where solar energy is not available," Ferlet-Cavrois said.  This was the case on nights that lasted 14 Earth days on the moon, and on expeditions to the solar system beyond Jupiter.  To survive the harsh lunar nights, China's active lunar rover Chang 'e-4 uses a plutonium heating device built in cooperation with Russia."  

Esa's first target for launching an americium power source is its Agonath lunar lander, scheduled for launch in the early 2030s.  "The Argonaut mission will conduct long-term research on the lunar surface and support astronauts working there," Landgraf said.  In the 2040s, ESA hopes to power missions to the ice giants Uranus and Neptune, Ferlet-Cavrois said.  These planets have only been studied during flybys by NASA's Voyager 2 probe in the 1980s."  

"Americium's availability, and the challenges of producing plutonium-238, means NASA may want to use it as well, and the agency is evaluating its ability to produce enough RTG for future missions," Landgraf said.  Of its Artemis project, which aims to establish a long-term presence on the moon, he said: "They think our americium is very interesting."  

"After more than a decade of research, americium technology has reached a stage where it can be developed for practical missions," Ambrosi said.  The excitement at the moment is actually palpable.  We've been working on this for a long time."