How Spacesuits Work: Pressure, Life Support, and Mobility

A spacesuit is a small, wearable spacecraft. It must hold a breathable atmosphere, protect its wearer from extreme temperatures and tiny high-speed particles, and still let a person bend, grip, and work for hours. Almost every design choice in a suit comes from a conflict between holding pressure and staying flexible.

Why a suit is needed

Space is a near-perfect vacuum, and the human body cannot function without outside pressure and oxygen. Exposure to vacuum causes swelling, a rapid loss of consciousness within roughly 15 seconds, and tissue damage from the lack of pressure. The body does not literally boil, but water vapor can form in tissues and gas can expand inside the body, which is why pressure must be maintained.

Sunlight and shadow add a second problem. Surfaces in direct sun can climb above 120 degrees Celsius, while shaded surfaces can fall below minus 150 degrees Celsius. A suit must keep its wearer comfortable across that range, and it must also block micrometeoroids and ultraviolet light.

Two families: IVA and EVA

Suits fall into two broad families. Intravehicular Activity (IVA) suits are worn inside a spacecraft and act as backup protection if cabin pressure is lost. Extravehicular Activity (EVA) suits are complete spacecraft for work outside a vehicle. They include their own oxygen supply, cooling, communications, and thermal layers.

Launch and entry suits

Crews wear pressure suits during launch and reentry because those are the phases when a cabin leak would be most dangerous. Soyuz crews have used the Sokol suit for this purpose, and Crew Dragon and Orion crews wear suits designed for emergency pressurization. These suits are lighter and less capable than EVA suits because they are not meant for long work outside.

Pressure and the gas choice

NASA's Extravehicular Mobility Unit, the EMU, runs at about 4.3 psi of pure oxygen, which is roughly 30 kilopascals. That pressure is far below sea-level air pressure of 14.7 psi. Lower pressure keeps the suit flexible, because a suit at full Earth-sea-level pressure would be nearly rigid and almost impossible to bend.

Pure oxygen at low pressure creates its own problem. Nitrogen dissolved in the body can form bubbles when pressure drops, causing decompression sickness, sometimes called the bends. To prevent that, astronauts breathe pure oxygen for several hours before an EVA. This pre-breathe purges nitrogen from the tissues and lowers the risk during the spacewalk.

Comparison of common suits

Suit	Role	Gas and pressure
EMU	Long EVA on the station	About 4.3 psi, pure oxygen
Apollo A7L	Lunar surface work	About 3.7 psi, pure oxygen
Sokol, Orion suits	Launch and entry backup	Emergency pressure, not EVA

The Apollo lunar suit operated at a slightly lower pressure than the EMU, which gave the astronauts more freedom to move on the Moon. Both designs rely on the same basic principle: a low-pressure, pure-oxygen environment inside a flexible shell.

Layers of the pressure garment

The EMU's pressure garment has several functional layers. The innermost layer is a gas-tight bladder that holds the pressurized oxygen. Outside it sits a restraint layer that controls the shape of the suit and carries the mechanical load of the pressure. Outermost is a thermal micrometeoroid garment, a set of insulating and protective layers that reduce heat flow and resist tiny particle impacts.

Taken together, the outer layers form several separate sheets of fabric and insulation. Each layer solves one problem, and each adds bulk and stiffness, which is why every new suit design is a careful balance of protection and movement.

Gloves and the hardest part

Gloves are often the most limiting part of a suit. A pressurized glove wants to puff outward, which forces the fingers to resist. Astronauts can experience hand fatigue after hours of gripping tools, and cramped fingers have caused real problems during past spacewalks. Engineers address this with joint designs, tighter fabric patterns, and special fingertip materials that allow grip while keeping the glove sealed.

Life support backpack

The EMU's backpack is called the Primary Life Support System, or PLSS. It stores the oxygen supply, circulates it through the suit, removes carbon dioxide, and carries the cooling system. Carbon dioxide is removed by lithium hydroxide canisters, which chemically bind the gas as the air passes through them. Fans move air across the helmet and through the canisters so the wearer does not rebreathe stale gas.

Cooling is one of the most important functions. Inside a suit, sweat cannot evaporate into still air, so the body would overheat quickly. A Liquid Cooling and Ventilation Garment, worn under the pressure suit, carries cool water through about 300 feet, or roughly 90 metres, of thin tubing woven across the torso and limbs. The heat moves from the body into the water, then into the backpack, where a sublimator releases it by turning ice directly into vapor in the vacuum of space.

The nominal duration of an EMU spacewalk is about seven hours of consumables, though actual EVAs are usually planned somewhat shorter to leave margin. Batteries, communications, and water supplies all have to be sized to that limit.

Mobility and the rigid suit problem

Mobility is the main design goal that engineers keep returning to. Every joint in a pressure suit must rotate against internal pressure, which pushes the limbs outward and makes them resist motion. Designers use bearings, convoluted joints, and carefully shaped fabric to reduce that resistance. Even so, bending a knee or raising an arm can take real effort after several hours of work.

Rigid or hard suits have been studied as a way to avoid the stiffness problem altogether. They use metal joints and an armored shell, which can keep the wearer at lower internal pressure with less effort. The trade-off is weight and bulk, and a rigid suit is harder to put on, harder to maintain, and harder to fit to many body sizes.

Communications and safety

Every EVA suit carries a radio link, camera systems, and emergency beacons. Crew members are tethered to the spacecraft and rely on ground controllers for guidance. Suits also monitor oxygen, carbon dioxide, pressure, and temperature, and they warn the wearer and the ground team when a value drifts out of range.

Training is as important as hardware. Astronauts practice spacewalks for hundreds of hours in large water tanks, where the buoyancy of the water simulates weightlessness while the suit remains pressurized.

Key takeaways

A spacesuit is a pressure vessel, a life support system, a thermal shield, and a set of joints that must bend. Its design is built around the trade-off between holding pressure and allowing movement.

The EMU's low-pressure, pure-oxygen design shows how engineers manage that trade-off. The pre-breathe protocol shows that the human body's own chemistry sets hard limits on how a suit can work.

For readers who want to understand what spacewalks are for, the station guide in this section describes the orbital laboratory that crews leave to repair and upgrade. For readers who want to understand what the suit protects against, the human-body article covers radiation, microgravity, and the long-term effects of living in space.