K-RadCube: The Artemis II Satellite That Runs on Water

The K-RadCube is about the size of a shoebox. It launched on Artemis II, NASA's first crewed lunar mission in over 50 years. And it runs on water. Not a metaphor. A water vapor propulsion system from UK-based SteamJet Space Systems heats liquid water and pushes the steam out. No chemicals, no fire, just thrust from a kettle.

Why water? NASA's safety rules for crewed missions are strict about explosion risks. Chemical fuels are toxic and dangerous to fly next to humans. Getting them certified would have taken too long. So the Korean team designed K-RadCube with a different solution from the start: an electrothermal thruster that turns water to steam.

How It Works

The Steam Thruster One uses about 20 watts of power, less than a typical laptop charger. It heats liquid water until it becomes superheated steam, then pushes that steam through a nozzle. Thrust comes out the back. Water is stored in low-pressure tanks, which greatly reduces the explosion risk.

The 12-Hour Burn

K-RadCube was deployed into a highly elliptical orbit with a perigee (closest point to Earth) dipping into the upper atmosphere. Without a correction, it would burn up on the first pass. The satellite was designed to perform a continuous 12-hour burn using its water thruster to raise that perigee to about 200 kilometers. That's one of the longest single burns ever attempted with a water-based propulsion system in space.

Where Things Stand

This is where it gets complicated. After deployment, ground stations detected signals from K-RadCube, but normal communication hasn't been established. The Korea AeroSpace Administration received partial signals and abnormal telemetry data, but they can't confirm whether the altitude-raising maneuver actually happened. If it didn't, the satellite may already be lost. If it did, they just can't talk to it properly. They're continuing attempts until April 4.

The Mission

K-RadCube's job is to fly repeatedly through the Van Allen radiation belts, measuring radiation levels and their effects on human tissue. It's also carrying memory chips from Samsung and SK hynix to test how they hold up in extreme space conditions. That data matters for every future crewed mission beyond low Earth orbit. The Moon. Mars. Anywhere past Earth's magnetic shield.

The water thruster itself isn't experimental. SteamJet has flown similar systems successfully since 2023 on other missions. But this is the first time a water-based system has been trusted with a critical, time-sensitive orbital correction on a crewed mission. Whether it worked or not, the fact that it was allowed to fly at all is the story. Water propulsion is safe enough to sit next to humans. That opens doors for every small satellite that comes after it.