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 an average 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-fission uranium would be less than a millirem, and more likely, 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 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.

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