Thermal control and the liquid cooling garment

Space is the ultimate heat management nightmare. On Earth, you sweat, the air moves, and you cool down. In space, you’re either broiling in direct sunlight at 250 degrees Fahrenheit or freezing in the shade at minus 250. Your spacesuit has to handle both extremes while keeping you alive and functional. That’s where thermal control systems come in, and the liquid cooling garment is the unsung hero of every spacewalk.

The liquid cooling garment, or LCG, is a tight-fitting body suit worn directly against the skin under the spacesuit’s hard outer shell. It looks like a cross between long underwear and a medical compression wrap, crisscrossed with thin plastic tubes. Water circulates through those tubes, pulling heat away from your body and dumping it into the suit’s cooling system. No fan, no refrigerant, no complicated phase-change materials—just water, tubing, and a pump. It’s brutal simplicity at its finest, and it has been the backbone of American spacesuit thermal control since the Apollo program.

Why water? Because water has an absurdly high specific heat capacity. It can absorb a massive amount of thermal energy without changing temperature much. That means a small volume of water can pull a lot of heat off your skin before it needs to be cooled down again. In a suit, every ounce of fluid matters because you’re already hauling oxygen, power, and life support. Water does the job without adding unnecessary mass.

The LCG works by direct contact. The suit fits snugly against your body, and the tubes are positioned to maximize coverage over heat-generating areas—your torso, thighs, arms. As your body works during a six-hour spacewalk, your metabolism cranks out heat. If that heat doesn’t leave, you overheat quickly. Hyperthermia in a suit isn’t just uncomfortable; it’s dangerous. Your cognitive function drops, your heart rate spikes, and you start making mistakes. The LCG prevents that by keeping your skin temperature constant, usually around 82 degrees Fahrenheit, regardless of what’s happening outside.

The suit’s backpack—the Portable Life Support System—does the heavy lifting. It contains a sublimator, which is a block of porous metal that vents water into space. As the water turns to vapor, it absorbs enormous amounts of heat, just like sweating cools you down on Earth. The chilled water then cycles back through the LCG, and the cycle repeats. This is not a closed-loop system in the true sense; you lose water to space every time you use it. That’s why suits carry a finite supply of water, typically enough for about six to eight hours of activity. Once it’s gone, you’re out of cooling capacity.

Modern iterations of the LCG have improved comfort and durability. Early Apollo suits used woven nylon tubes that could kink or leak. Today’s suits use flexible polyurethane tubing with welded joints. NASA’s current EMU (Extravehicular Mobility Unit) uses a garment with about 300 feet of tubing sewn into a form-fitting Spandex and nylon layer. It breathes, wicks moisture, and allows full range of motion. But it still has limitations. The suit can only dump heat through sublimation, which means it doesn’t work well in a vacuum if the water supply runs low. And the garment itself is custom-fit for each astronaut, which is expensive and time-consuming to produce.

For future missions to the Moon and Mars, engineers are rethinking the LCG entirely. NASA’s new xEMU suit, designed for Artemis, uses a more advanced thermal control system that combines liquid cooling with phase-change materials embedded in the fabric. These materials absorb heat without needing as much water, extending mission duration and reducing resupply needs. On Mars, where water is scarce, that’s a game changer. The suit will also use variable flow control, letting the astronaut or ground control adjust cooling rates in real time based on workload and external temperature.

Commercial spaceflight companies are also developing their own liquid cooling garments. SpaceX’s IVA suits, used inside the Crew Dragon, don’t need full thermal control because the cabin is climate-controlled. But for future extravehicular activities on Starship, they’ll need something far more robust. Same goes for Axiom Space, which is building commercial suits for private space station operations. Their designs are modular, aiming to fit a wider range of body types without custom tailoring.

The LCG is a classic example of engineering elegance. It does one thing—remove heat—and does it with the simplest possible mechanism. No exotic materials, no complex electronics, no moving parts except a pump. It’s reliable because it has to be. On a spacewalk, there is no backup. If the cooling fails, you have minutes before you have to abort. That’s the reality of working in a place where the difference between comfort and death is a few degrees.

For anyone serious about understanding how we survive off-planet, the liquid cooling garment is the piece of tech you can’t ignore. It’s not flashy. It doesn’t make headlines. But without it, no astronaut leaves the airlock. Period.