Astronauts living on the Moon will need a lot of power, but they can't bring a steady supply of fuel with them.
Like many other science fiction shows, the 1970s TV series Space: 1999 was set after a nuclear explosion blasted the Moon out of Earth's orbit and sent the lunar base Alpha, and the scientists working there, on a journey through deep space.
It clearly made an impression on a young Elon Musk, who in 2017 named SpaceX's proposed lunar outpost Alpha when outlining his plans for a future Moon base. Today, SpaceX is working with NASA to return humans to the lunar surface as part of the Artemis program.
NASA and the US State Department have drawn up guidelines for the peaceful exploration of the Moon in the form of the Artemis Accords. So far, 36 nations have signed up, including India, Japan, the UK, Canada, Australia, the United Arab Emirates, and South Korea.
China also has lunar base ambitions. The International Lunar Research Station, proposed in 2021, now has Russia, Belarus, Pakistan, Azerbaijan, Venezuela, Egypt, and South Africa signed up as partners.
Any nation or alliance that builds the first base on the Moon will need a reliable power source, something that has not escaped the notice of space agencies and companies around the world.
"The reality is, nuclear is the only option for providing baseload power on the Moon," says Simon Middleburgh of the Future Nuclear Futures department at Bangor University in Wales.
A lunar day is not 24 hours as on Earth, it is a month, 29.5 Earth days to be precise. That means two weeks of sunlight, then two weeks of darkness, with temperatures dropping to -130C.
This is why the Apollo missions between 1969 and 1972 all took place on the Moon's equator, when temperatures were manageable and there was constant sunlight to power the science experiments and rovers.
At the lunar south pole, where one of the most likely locations for a base lies, certain spots get sunlight for over 80 percent of the time. But temperatures can drop even lower in permanently shadowed craters, where water ice can be found—essential for not only keeping astronauts alive, but also for producing fuel, as there is no gas or oil on the Moon.
"Nuclear is the only show in town on the Moon, you can't take enough fuel with you, solar panels aren't going to cut it, diesel generators aren't going to cut it and the old radioisotope thermal generators aren't big enough to provide the power," says Middleburgh.
The first radioisotope thermal generator was used on the Moon in 1969 by Apollo 11, using the heat generated by the radioactive decay of plutonium-238 to keep scientific instruments warm enough to operate. On Apollo 12, this heat was converted into electricity to power an experiments package, marking the first use of a nuclear reactor on the Moon, albeit not on the scale of those on Earth. This cylindrical generator measured just 45.7 by 40.6 centimeters.
Any nuclear reactor sent to the Moon will need to be light enough and robust enough to survive a 240,000-mile journey, then be deployed in the harsh conditions on the lunar surface, including abrasive dust and radiation.
In 2022, NASA awarded contracts to Lockheed Martin, Westinghouse, and IX, working with Intuitive Machines and X-Energy, to develop nuclear power plants for the Moon.
The first phase was completed in February, with the companies presenting designs for a reactor that could power a lunar outpost for at least a decade.
"We're leveraging nuclear technologies used on previous space missions such as Pioneer, Voyager, and Cassini, where these systems far outlasted their original design life," says Shatel Bhakta, lead lunar architect at NASA's Johnson Space Center.
He says the design of a surface nuclear power source takes into account the harsh environment, the need to minimize mass and volume, the requirement for high reliability to ensure a continuous source of electricity to support the crew, and, because of the distance from Earth and the time delay in communications, the system must be designed to be autonomous, with minimal human intervention.
Meanwhile, in March, Russia's Roscosmos space agency announced plans to build a nuclear power plant on the Moon with the China National Space Administration by 2035, to provide power for their joint lunar research station.
Roscosmos Director General Yury Borisov told Russian state media that the station would be built "without the participation of people."
The UK Space Agency has also announced £2.5m of new funding to design a modular nuclear reactor for the Moon. For over 60 years, Rolls-Royce has quietly designed, manufactured, and supported all the nuclear reactors that power the UK Royal Navy's submarines.
"We have this heritage of delivering very compact nuclear reactors, so we're taking that capability into these really exciting new areas like space exploration," says Jake Thompson, chief engineer for new nuclear programs at Rolls-Royce.
Rolls-Royce's Micro-Reactor Program is in the concept development phase, with testing currently being carried out on prototype components, the aim being to have a design ready to go to the Moon by 2029.
"These are fission based reactor systems, so they will be using low enriched uranium, we have a pretty good idea of what these systems will look like and—critically for space—how much they will weigh," says Thompson. Each Rolls-Royce microreactor will generate 50-100kW of power and last for at least a decade.
He says it depends on the architectural and infrastructure demands on the lunar surface, but envisions a microgrid with a couple of reactors supplemented by solar at the south pole.
The microreactors would be about the size of a family car and weigh a few tonnes, he says. That's very small for a nuclear reactor, but still relatively large when it comes to a space system. Small reactors are seen as key to a successful design by many organizations, including the Nuclear Futures Institute, which is collaborating with Rolls-Royce on the project.
"We will only deploy a system when it is safe in every respect, that includes the launch and the reactor is only designed to go critical once it is actually on the lunar surface, so before the reactor is turned on, the nuclear fuel inside it is in an inert form, so it is perfectly safe to handle, touch, and it is not radioactive until the reactor is turned on," he says.
As part of the design process, engineers are also considering the end-of-life arrangements for these microreactors.
"When our lunar reactor mission is ended, we will shut it down and the radioactivity will decay to a point where it can be safely approached and transferred to a long-term storage location if desired," says Bhakta.
Funding and time will be needed for these technologies to mature, but the applications for microreactor designs could extend from the Moon back to Earth—everything from flexible power modules for remote locations, which could be orders of magnitude smaller than existing power plants, to nuclear medicine.
Middleburgh is optimistic about the technologies both in space and on Earth, arguing that nuclear has had a number of renaissances, but needs a chance to demonstrate that it can deliver reliable, carbon-free power on demand.
"These applications are fantastic if we can show people that nuclear can be delivered on time, on budget, and doing really useful things—things that will save the world," he says.