Artist's concept of a Fission Surface Power system on the Moon.
Credit: NASA
It's an ambitious plan of NASA's to return astronauts to the Moon by 2020 and set up a lunar outpost.
Just one of the many challenges their engineers face is finding a way to power this most remote manned beacon of human civilisation with nothing to burn, little sunlight, no running water and no wind.
One solution, though, could be nuclear power.
Fission Surface Power (FSP) is one of the more interesting options NASA is considering. If this method is chosen, an engine invented in the early 1800s by Scottish brothers Robert and James Stirling could help make it work.
Over the years the Stirling engine has earned a sterling reputation here on Earth, and it may one day prove its worth on the Moon.
"Inhabitants of a lunar outpost will need a safe and effective way to generate light and heat and electricity," says Mike Houts of NASA's Marshall Space Flight Centre. "The tried and true Stirling engine fits the bill. It's not only reliable and efficient, but also versatile and clean."
NASA is partnering with the U.S. Department of Energy to develop Fission Surface Power technology to produce heat and feed it into a Stirling engine, which, in turn, would convert heat energy into electricity for use by Moon explorers.
It's not certain that NASA will adopt this kind of power system, but it does have some very appealing qualities, says Houts.
"A key advantage to this power system is that it wouldn't need sunlight to operate. An FSP system could be used to provide power any time, any place, on the surface of Moon or Mars.
"It could be used at the poles and away from the poles, it could weather a cold lunar night, and it would do well in places like deep craters that are always shaded. Not even a swirling, sunlight-obscuring Martian dust storm could stop it," he says.
NASA's engine would only need to produce 40 kW or less power – just enough for a lunar outpost.
"This power level is high by space standards but extremely low by Earthly standards," says Houts. "It's about one 20,000th of what a typical Earthly reactor puts out. We'd only need a tiny reactor on the Moon – the fuelled portion would be only about 10 inches [25 cm] wide by 1.5 feet [46 cm] long."
It would provide more power with less mass than other power systems. The whole assembly – radiator on top of Stirling engine on top of reactor – could be stowed in a fraction of the lunar lander.
Before developing the final system, Houts and his team are testing with non-nuclear power for proof of concept.
"We're conducting tests in a thermal vacuum to learn about operating and controlling the system on the Moon," he says. "We're using resistance heaters to simulate nuclear heat. Electrical resistance produces heat."
After the test system proves the viability of the concept, the team could be directed to build the real thing, drawing heavily on U.S. and international terrestrial reactor experience.
"It would be built from stainless steel and fuelled by uranium dioxide. This combination has been used in terrestrial reactors throughout the world, so scientists and engineers are well versed in its operation."
The unit would not be active at launch, but would be turned on once in place on the lunar surface, where it would be surrounded by shielding to prevent any hazard from the radiation emitted.
"It would be very safe," says Houts. "And the beauty of the system is that it would be practically self-regulating."
Here's how it would work: Inside the reactor is a bundle of small tubes filled with uranium. Outside the reactor are control drums – one side of each drum reflects neutrons and the other side absorbs them, providing a way to control the rate that neutrons escaping the reactor core are reflected back in.
To start up the unit, the absorbent side of each control drum is turned out, away from the reactor core, so the reflective material faces in and sends escaping neutrons back in to the core. The resulting increase in available neutrons enables a self-sustaining chain reaction, which produces heat.
A coolant (sodium potassium mixture) flows through the passageways between the tubes, picks up the thermal heat produced by the reacting uranium, and transfers the heat to the Stirling engine. The Stirling engine then does its magic to generate electricity.
Meanwhile the coolant, which has 'downloaded' some of its cargo (heat) to the Stirling engine, circulates back through the reactor core, where it picks up heat and is ready to repeat the entire cycle.
The system would use only a minuscule amount of fuel – one kilogram of uranium every 15 years – and still have enough reactivity to run for decades.
"We give it a life expectancy of eight years, though, because something else would falter before the fuel would run out," says Houts.
After shutdown, radiation emitted by the system would decrease rapidly. A replacement system could easily be installed at the same site, and may be able to power not just outposts on the Moon, but Mars and future destinations as well.
Dauna Coulter is a writer for the U.S. space agency, NASA.
This is an edited version of a feature first published on the Science@NASA web site.
