Most space missions rely on solar power, but a few that demand maximum reliability have a thermal nuclear power system. Human spaceflight and colonization efforts will require a lot of power that works no matter the conditions. That’s why NASA has been developing small-scale nuclear fission reactors. The space agency now reports that the delightfully named KRUSTY (Kilopower Reactor Using Stirling Technology) reactor has passed a vital ground test. This could be the first prototype of reactors that power future human settlements on the moon or Mars.

NASA isn’t looking at sending nuclear reactors into space just for fun — it’s simply the best option available to us. Solar power is a cheap and easy way to power a spacecraft, but the efficiency drops off sharply as you get farther from the sun. On Mars, for example, solar panels produce much less power than they do on Earth. The radio thermal generators used on rovers like Curiosity are more reliable, as they convert heat from radioactive decay directly into electricity. However, they don’t produce very much power.

A Kilopower reactor like KRUSTY is designed to generate up to 10 kilowatts of electricity, which is enough to run several typical households. NASA estimates four such reactors would enough to power a human outpost on the moon or Mars, and they reactors would operate for at least 10 years before needing more nuclear fuel.

In cooperation with the National Nuclear Security Administration (NNSA), NASA has successfully tested the reactor at a Nevada facility in four distinct phases. The first two were conducted without power to ensure the physical components operated as expected. Then, the core was powered up in stages to ensure it remained stable. In the final phase, NASA cranked the KRUSTY to full power for 28 hours to simulate operation on a real mission.

The reactor uses uranium-235 fuel, which generates heat as it undergoes nuclear fission. Passive sodium heat pipes transfer heat into a high-efficiency Stirling engine, a closed-cycle engine that uses expansion and compression of gas to convert the heat into power. NASA says the reactor withstood simulated power reductions, failed engines, and failed heat pipes during testing. The system is self-regulating, so no one needs to sit at a control panel all day watching it.

KRUSTY is currently part of the Space Technology Mission Directorate (STMD) in Washington. As the technology nears readiness, it will be transferred to the Technology Demonstration Mission around 2020.