A spacesuit is not protective clothing. It is a person-shaped spacecraft: pressurised, thermally regulated, radiation-shielded to a modest extent, with its own atmosphere, power, water, cooling, and communications. Making one that a human can work in for eight hours without catastrophic fatigue is one of the hardest ongoing engineering problems in human spaceflight.
What a Suit Actually Does
An Extravehicular Mobility Unit (EMU) — the technical name for what non-specialists call a spacesuit — has to do at least six things simultaneously:
- Pressure. Maintain an internal pressure high enough to keep the wearer conscious and prevent decompression sickness, while allowing joint movement. Current U.S. and Russian suits operate well below sea-level pressure — 4.3 psi for the EMU, 5.8 psi for Orlan — which requires long pre-breathe protocols on pure oxygen to purge nitrogen from the body.
- Breathing gas. Supply oxygen, scrub carbon dioxide (using lithium hydroxide cartridges or metal-oxide absorbers), and control humidity.
- Thermal regulation. Manage body heat through a liquid-cooled garment — a long-underwear-like layer with cold water circulating through tubing — while the external suit handles solar heating (extreme on the day side) and radiative cooling (extreme in shadow).
- Power and comms. Run fans, pumps, heaters, radios, lights, helmet cameras, and biomedical telemetry for the duration of the EVA.
- Mobility. Allow the wearer to bend joints against the suit's internal pressure. Every joint in the suit has to work like a constant-volume mechanism so the wearer is not fighting a balloon trying to straighten out.
- Debris and radiation shielding. Provide enough mass and layering to stop micrometeoroids and some ionising radiation.
Any one of these is a solvable engineering problem. Doing all of them at the same time in a garment a human can wear for an entire workday is the hard part.
EMU (NASA/ISS)
The EMU is the U.S. EVA suit that has been in service on the Shuttle and ISS programmes since 1981. It is a hard upper torso with soft-goods arms and legs, a bubble helmet, and a backpack-style Primary Life Support System (PLSS) containing the oxygen tank, water tank, CO2 scrubber, battery, and radio.
The EMU is ageing. It was never designed for the 20-plus-year extended operation it has seen. Suit availability has been a chronic problem during long ISS expeditions, and incidents like the 2013 Luca Parmitano water-in-helmet event have underlined that the suit's thermal-control loop is difficult to maintain. NASA has been trying to replace or significantly upgrade the EMU for most of the ISS era; several contracts have come and gone without a flight-ready successor.
Orlan (Russian)
Orlan is the Russian EVA suit used on the Mir space station and the ISS Russian segment. Its most distinctive feature is that the wearer enters through a rear hatch — the back of the suit swings open like a door, the cosmonaut climbs in feet first, then closes the hatch. Donning takes a fraction of the time needed for the EMU, at the cost of some suit mass and complexity.
Orlan is conceptually similar to the EMU in most other respects: rigid torso, soft limbs, backpack life support, similar EVA duration. It operates at a higher internal pressure (5.8 psi vs. the EMU's 4.3 psi), which reduces the required pre-breathe time.
AxEMU (Axiom Space for Artemis)
The Axiom Extravehicular Mobility Unit (AxEMU) is the spacesuit being developed by Axiom Space for Artemis lunar surface missions, under a NASA services contract awarded in 2022. AxEMU is a new-clean-sheet design, not a modification of the EMU, though it draws on xEMU technology developed earlier at NASA.
AxEMU differs from the EMU in several ways relevant to lunar surface use:
- Greater joint mobility. Bending, kneeling, and squatting on the lunar surface require much more flexibility than microgravity EVAs. The suit has redesigned hip, knee, and shoulder joints.
- Wider size range. The EMU's size options were notoriously limited, leading to the cancellation of the first all-female spacewalk in 2019 when two suits in the right size were unavailable. AxEMU is designed to fit a much broader range of body sizes from the start.
- Dust tolerance. Lunar regolith is abrasive and electrostatically clingy. AxEMU seals, bearings, and external surfaces are designed to survive repeated surface EVAs without the dust-ingress problems that plagued Apollo suits after even short exposures.
- Thermal environment. Lunar surface temperatures swing between day and night by nearly 300 °C. The suit has upgraded thermal regulation to handle both extremes, particularly for operations in permanently shadowed regions near the south pole.
AxEMU is one of the critical-path items for Artemis III. Without a flight-ready lunar suit, even a successful HLS landing cannot actually put astronauts on the surface.
Launch and Entry Suits
EVA suits are distinct from launch-entry suits, which are worn inside the spacecraft during ascent and re-entry but are not designed for walking in vacuum. These include the Russian Sokol suit used on Soyuz, SpaceX's in-house flight suit used on Crew Dragon, the Boeing Blue suit developed for Starliner, and various orange "pumpkin suits" that preceded them on the Shuttle. These suits provide survival capability in case of cabin depressurisation, not EVA capability. The distinction matters: the engineering problems for a launch-entry suit are much easier than for a full EMU.