An EVA suit, short for Extravehicular Activity suit, is a wearable spacecraft that keeps an astronaut alive outside a vehicle in the vacuum of space or on the surface of another world. It supplies oxygen, removes carbon dioxide, regulates temperature across a 500-degree Fahrenheit swing, and provides enough mobility for an astronaut to work with tools, collect samples, or make repairs. Every EVA suit is essentially a self-contained life support system shaped like clothing.
How an EVA Suit Differs From a Pressure Suit
The earliest American spacesuits, used during the Mercury and Gemini programs, were adapted from military aviation pressure suits. They were air-cooled, offered minimal mobility, and existed only as a backup: if the cabin lost pressure, the suit would keep the astronaut alive long enough to operate spacecraft controls. These suits were never meant for working outside.
An EVA suit goes far beyond that. It must function as an independent spacecraft for hours at a time, providing breathable atmosphere, thermal regulation, radiation shielding, micrometeorite protection, and communications. NASA calls its version the Extravehicular Mobility Unit, or EMU. The name captures the idea: it is a mobile unit, not just a garment.
Major Components
The current NASA EMU has two main parts: the spacesuit assembly (SSA) and a backpack called the Portable Life Support System, or PLSS. The SSA is built from multiple layers of fabric and flexible materials attached to a rigid fiberglass piece called the hard upper torso. This fiberglass shell is the structural backbone of the entire suit. The helmet, arms, lower torso, and the PLSS backpack all mount directly to it.
Beneath the outer suit, the astronaut wears a liquid cooling garment that looks like long underwear laced with thin tubing. Water circulates through this tubing, absorbing body heat and carrying it to a device in the backpack called a sublimator, which vents the heat into space. Without this system, an astronaut’s own body heat would overwhelm the suit within minutes.
The PLSS backpack handles everything else: oxygen supply, carbon dioxide removal, battery power, water circulation, and radio communications. Oxygen tanks are charged to about 3,000 psi of pure oxygen. Carbon dioxide is scrubbed from the breathing air using a chemical swing bed that captures CO2 on one cycle and vents it on the next, keeping the air inside the suit breathable for the duration of a spacewalk. For spacewalks near the International Space Station, the suit can also carry a small jet-propulsion device called SAFER, a last-resort backpack that lets an astronaut fly back to the station if they become untethered.
Surviving the Vacuum
Space has no air pressure, and without a pressurized suit, the gases dissolved in an astronaut’s blood would form bubbles, a condition similar to what scuba divers call “the bends.” The EMU maintains an internal pressure of 4.3 psi using 100% oxygen. That is far lower than the 14.7 psi of sea-level air on Earth, but it delivers enough oxygen to keep the body functioning normally.
The tradeoff with pressure is mobility. Higher pressure makes the suit stiffer, like an over-inflated balloon, making it harder to bend joints and grip tools. Lower pressure improves flexibility but increases the risk of decompression sickness during the transition from the station’s cabin atmosphere. To manage this, astronauts go through a pre-breathe protocol before every spacewalk, spending time breathing pure oxygen to flush nitrogen from their bloodstream before stepping into the lower-pressure suit.
Newer suits for the Artemis lunar program are being designed to operate at higher internal pressures, which shortens that pre-breathe time. The result is less waiting and more time actually working on the lunar surface.
Temperature and Radiation Protection
In orbit, an astronaut can swing from 250°F in direct sunlight to negative 250°F in shadow, sometimes within seconds as the station rotates. The suit’s outer layers use reflective materials and insulation to buffer these extremes, while the liquid cooling garment handles heat from the inside. The combination keeps the astronaut’s skin temperature comfortable even as the suit’s exterior surface experiences conditions that would instantly burn or freeze exposed flesh.
The layered construction also provides a barrier against micrometeorites, tiny particles of space debris traveling at thousands of miles per hour. No single layer stops everything. Instead, the suit relies on a sandwich of different materials, each absorbing or deflecting a portion of the impact energy.
Weight and Mobility Challenges
A fully loaded EMU weighs roughly 130 pounds on Earth without an astronaut inside it. The Apollo-era suits ranged from about 147 to 212 pounds depending on the version and whether water and consumables were included. In microgravity on the ISS, that weight is irrelevant since nothing feels heavy. On the Moon, where gravity is one-sixth of Earth’s, a 212-pound suit weighs only about 35 pounds, and the suit’s internal pressure actually helps support itself. Research has shown that at lunar gravity, the pressure force pushing through the suit’s legs exceeds the suit’s own weight by nearly three times, meaning the suit essentially holds itself up.
Still, mass creates inertia. Even in zero gravity, swinging a 130-pound suit around requires real effort, and every movement carries momentum that must be controlled. Astronauts consistently report that spacewalks are among the most physically demanding tasks they perform. Gripping tools against the resistance of pressurized gloves is especially tiring on the hands and forearms.
Next-Generation Suits for the Moon
NASA’s current spacewalk suits were designed in the 1980s for work in Earth orbit, not for walking on another world. For the Artemis program, which aims to return astronauts to the lunar surface, Axiom Space is developing a new suit called the AxEMU (Axiom Extravehicular Mobility Unit). It is built with significantly improved joint mobility, including the ability to bend down and pick up geology samples, something the stiff Apollo suits made extremely difficult.
The AxEMU also features increased sizing options and adjustability to fit a wider range of body types, addressing a long-standing limitation of the legacy EMU, which was effectively built around a narrow range of male body dimensions. Advanced life support systems and enhanced protection for the harsher lunar environment, where dust, radiation, and temperature swings differ from low Earth orbit, are central to the design. Axiom is also developing specialized surface tools that integrate with the suit, making sample collection and scientific tasks more practical than they were during Apollo.
Higher operating pressures in the AxEMU reduce pre-breathe time, which directly translates to longer productive moonwalks. During Apollo, surface excursions were tightly limited. The new suits aim to let astronauts spend substantially more time outside the lander on each outing.