The specialized garments worn by astronauts underneath the bulky Extravehicular Mobility Unit (EMU) are a sophisticated system designed for survival and function. These underlayers are necessary because the rigid, pressurized outer suit creates an isolated, personalized environment that must be controlled. The vacuum of space and the physical exertion of a spacewalk, or Extravehicular Activity (EVA), present extremes of heat, cold, and biological needs that must be managed internally. This collection of specialized clothing, sensors, and communication gear ensures the wearer remains healthy, comfortable, and connected while working outside the spacecraft.
The Skin-Contact Base Layer
The innermost layer against the astronaut’s body is a lightweight garment, often made from a moisture-wicking synthetic fabric. This base layer helps to absorb perspiration and prevents the technical layers above it from causing skin irritation during long periods of use.
Directly outside this comfort layer is the Maximum Absorbency Garment (MAG), which manages human waste during extended operations. Spacewalks can routinely last eight hours or more, and the astronaut cannot exit the suit to use the toilet during that time.
The MAG functions like a high-tech adult diaper, designed to handle liquid waste for the duration of the EVA. It incorporates a super-absorbent polymer, such as sodium polyacrylate, capable of absorbing hundreds of times its own weight in liquid. This polymer transforms the liquid into a solidified gel, which locks the moisture away and prevents irritation. The garment provides a sanitary solution for biological needs, allowing the astronaut to focus on tasks outside the International Space Station.
The Active Thermal Regulation Garment
The Liquid Cooling and Ventilation Garment (LCVG) is the most complex layer, serving as the primary system for regulating the astronaut’s body temperature. This full-body suit is constructed from a form-fitting, stretchy fabric, like Spandex, with a dense network of thin, flexible tubing woven throughout the material. Chilled water circulates through this extensive network, which can contain approximately 300 feet of tubing, running close to the astronaut’s skin. The circulated water draws away the metabolic heat generated by the astronaut’s physical labor during the EVA.
This heat transfer is required because the outer spacesuit is well-insulated, designed to protect against the extreme cold of space, but it also traps body heat inside. Without the constant removal of this internal heat, the astronaut would quickly overheat, a condition known as hyperthermia.
The water flows from the LCVG and plugs into the Portable Life Support System (PLSS), the large backpack component of the suit. Inside the PLSS, a heat exchanger cools the water before it is pumped back into the garment to repeat the cooling cycle.
The LCVG also performs a ventilation function through ducts that draw air from the suit’s extremities. This circulated air removes moisture and exhaled carbon dioxide from the microclimate inside the suit, especially around the helmet area. The combination of liquid cooling and air ventilation ensures the astronaut is not only cool but also dry, maintaining a stable internal environment.
Integrated Monitoring and Communication
Layered over the cooling garment is the specialized headgear required for connectivity and health monitoring. The primary communication device is the Communications Carrier Assembly (CCA), commonly nicknamed the “Snoopy Cap” because of its resemblance to the cartoon character’s aviator headwear. This soft, fitted fabric cap is worn directly on the head beneath the hard helmet. It integrates a headset with earphones and boom microphones positioned near the mouth.
The CCA enables voice transmission, connecting the astronaut to their crewmates and Mission Control on Earth for instructions and coordination.
The underlayers also facilitate biological monitoring. Biomedical sensors, such as EKG electrodes, are affixed to the astronaut’s torso underneath the suit layers. These sensors transmit vital signs, including heart rate, back to flight surgeons. Flight surgeons use the data to assess the astronaut’s physiological state throughout the spacewalk, ensuring their well-being is managed while they operate outside the spacecraft.