The Kilopower fission reactor will offer a more efficient and more powerful portable power source for solar-system exploration.

An artist's conception shows Kilopower fission-reactor units on the surface of Mars.

NASA announced a new style of nuclear generator last week, one that may become a permanent fixture on lunar outposts or deep-space missions in the coming decades.

A dependable power source is the name of the game in solar-system exploration. Here among the inner planets, there's ample power to be had in the form of solar radiation. But this power drops off by the inverse square of the distance to the Sun. NASA's Juno mission to Jupiter, for example, was the first spacecraft to venture beyond the asteroid belt using solar energy, and it needed three huge, school-bus-size solar panels to do it.

More typically, venturing into the outer solar system has required nuclear power. Missions have long used Radioisotope Thermoelectric Generators (RTGs) — and the current-model Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs). But these use heat from the decay of plutonium-238, which is in limited supply, and they provide less than 200 watts of electricity. That's enough to power a roving robot but hardly enough for a colony.

With an eye to the future, NASA is developing Kilopower, a small fission reactor that's capable of generating a continuous output of 10 kilowatts of electricity for a minimum of 10 years — more than enough to run several average American households.

“We want a power source that can handle extreme environments,” says Lee Mason (NASA). “Kilopower opens up the full surface of Mars, including the northern latitudes where water may reside.” Portable nuclear power would also be ideal for exploring the permanently shadowed polar craters on the Moon.

stirling engines
A close-up of the nuclear Stirling engines being developed for long-duration space exploration.
NASA / Glenn Research Center

NASA's Glenn Research Center developed the kilowatt prototype in collaboration with the Los Alamos National Laboratory. Engineers deemed the project feasible in 2012 and have since been moving toward a full-scale demonstration. The uranium reactor core was supplied by the Y12 National Security Complex, and the entire prototype assembly was shipped to the Nevada National Security Site for early testing late last year. This will culminate with a 28-hour, full-power test in late March.

Kilopower would open up areas of the inner solar system to long-term exploration as well. On the Moon, for example, night is two weeks long. And on Mars, sandstorms periodically cover the solar panels used by rovers such as Spirit and Opportunity. For this reason, Curiosity uses a plutonium-powered MMRTG, as will the Mars 2020 rover.

Nukes in Space

Launching nuclear generators into space isn't without its issues. The U.S. lost one of its very first orbit-bound generators, which burned up over the Indian Ocean shortly after launch in 1964. NASA also faced an unexpected dilemma when the Apollo 13 crew returned to Earth with the nuclear-powered Apollo Lunar Surface Experiments Package, which was meant to remain on the Moon. It was ultimately ditched over the Marianas Trench in the Pacific Ocean, along with the Aquarius lunar module-turned-lifeboat.

However, tests show that Kilopower doesn't pose a threat. The average American receives about of 620 millirems per year cumulative from background radiation; if a Kilopower reactor were lost and the core breached during a launch, the peak dose from exposure to un-fissioned uranium would be less than a millirem, and would more likely be in the microrem range, according to Pat McClure (Los Alamos National Laboratory).

Previous launches incorporating plutonium including New Horizons, Curiosity and Cassini drew a scattering of protestors to the Florida Space Coast. Cassini in particular raised some concern, as it also performed an additional Earth flyby enroute to Saturn.

Kilopower vs. MMRTGs

Kilopower generates energy from uranium fission, a change from the plutonium-238 used by MMRTGs. The heavily regulated plutonium-238 is currently in short supply, as the U.S. Department of Energy only recently restarted the production pipeline for space exploration.

Kilopower reactors make use of Stirling engines, which compress and expand a fluid (in this case, liquid metal) in order to convert heat from uranium fission into mechanical power. This power can then run a generator and produce electricity. NASA had shelved similar Stirling technology during the lean fiscal times of 2013, but now it's back on the table. Stirling engines are at least four times more efficient than traditional MMRTGs.

Planned future missions may sport Kilopower technology, including proposed orbiters for the ice giants, Uranus and Neptune, and a nuclear-powered drone to explore Titan. NASA's Deep Space Gateway might also end up using Kilopower for its lunar surface operations. Perhaps the new nuclear age will be in the realm of space exploration.




Image of Lindsay


January 26, 2018 at 8:21 pm

Thanks for that info David. I don't remember hearing about Apollo 13's nuclear power plant. Will Kilopower reactors require Congressional approval for launch?

You must be logged in to post a comment.

Image of DukeBriscoe


January 26, 2018 at 10:13 pm

I think this article misinterpreted some of the details : "Kilopower reactors make use of Stirling engines, which compress and expand a fluid (in this case, liquid metal) in order to convert heat from uranium fission into mechanical power." That did not sound right to me (using expansion of liquid metal to drive a Stirling engine), so I spent a good bit of time trying to find more details. I did not find a really clear source, but the ones that tangentially mention Kilopower and similar NASA work are , , and

The one thing that seems definite is that the liquid metal (NaK) is used in a "heat pipe" which conducts heat to the Stirling engine. From there, there seems in some systems to be an intermediate water pipe system for further conducting heat to the Stirling engine pistons, and helium gas is the fluid inside the pistons. There is reference to the helium gas also serving as a "gas bearing" which reduces friction wear within the pistons.

You must be logged in to post a comment.

Image of MurphreeOkieInOKC


January 29, 2018 at 10:18 pm

Thanks Duke, the nasa article was really helpful. The overall flow 1) positive feedback neutrons creating more neutrons reaction creating heat at 800 Kelvin 2) liquid sodium metal heat pipes. Carrying heat to heated plate with sterling engine pistons with one end tied to the heat plate and one end connected to big palm leaf heat radiators that use radiative heat transfer to space, sink.3) the heat pipes and radiating leaves create different temperatures on opposite end of the piston of sterling engine 4) I think that helium is the working fluid of the sterling engine and the Glenn center mentions it. 5) the sterling piston has a magnet or generates a magnetic field and current just like an electric genetator in your car.
I vote for moon rover 1) quick development time 2) legacy mars rover + 10 kw power makes USA look like top dog in lunar space race 3) polar ice to rrocket fuel

You must be logged in to post a comment.

Image of David Dickinson

David Dickinson

January 27, 2018 at 8:37 am

Good research question... unsure, the only precedent for a uranium fission reactor launch vs a plutonium RTG by the U.S. was SNAPSHOT back in the 1960s.

You must be logged in to post a comment.

Image of MurphreeOkieInOKC


January 27, 2018 at 3:32 pm

I) ion power engines speed increases inside Jupiter, and works everywhere like outside Pluto
II) rovers to me always seem power starved: activities must be severely limited. By schedule, motor strength. Speed. We can now do 3 instruments at a time for decades.
Robert murphree

You must be logged in to post a comment.

You must be logged in to post a comment.