EVA spacewalk suits
An EVA suit is a one-person spacecraft — pressure shell, closed-loop life support, thermal control, sun visor, and a thruster pack — built so a human can spend 6–8 hours outside in vacuum.
The EVA suit's central problem is **closed-loop rebreathing**. Outside the airlock there is no atmosphere to vent CO₂ into, so the suit must scrub it from the astronaut's exhalation and recycle the remaining oxygen. The current US EMU runs a **lithium hydroxide (LiOH) canister** in the backpack that absorbs CO₂ irreversibly — replaced ground-side after every EVA. The Russian Orlan and the latest US METOX canister use a **regenerable amine swing-bed**: it adsorbs CO₂ during the EVA, then is baked out in the airlock between walks and reused. The station-side equivalent inside the ISS (the CDRA, Carbon Dioxide Removal Assembly) does the same thing on a regenerative cycle, while the OGS (Oxygen Generation System) electrolyses water to replenish O₂. The suit is a miniature, hand-portable version of station ECLSS, running for 8 hours on its own power and supplies.
The Primary Life Support System (**PLSS**) backpack carries oxygen at ~6800 kPa (1000 psi) in two tanks, a battery, the LiOH canister, a fan to circulate air through the helmet, a water tank for cooling, and a sublimator — a porous plate that lets liquid water boil into vacuum, dumping ~250 W of metabolic heat to space as water vapour. A liquid-cooling garment is worn against the skin: water threading through tubes against the body picks up heat and returns to the sublimator. The arms and gloves get pressurised gloves; the body gets a Hard Upper Torso (HUT) made of fibreglass or composite; the legs articulate through soft fabric joints. Operating pressure is 4.3 psi (US EMU, pure O₂) or 5.7 psi (Russian Orlan, pure O₂). Lower pressure = easier to move; higher pressure = less pre-breathe time.
Two specialised details. **Sun visor**: the gold-plated outer visor on every EVA helmet is gold for a reason — a thin vapour-deposited gold film reflects ~98% of infrared radiation while letting visible light through. With the Sun in the suit's field of view (about half the orbit on the ISS), direct sunlight on a clear polycarbonate visor would heat the helmet beyond tolerable. The astronaut flips the gold visor down when looking sunward and back up when working in shadow. **EVA thrusters**: the SAFER (Simplified Aid For EVA Rescue) backpack on every modern US EVA carries 1.4 kg of compressed nitrogen and 24 small thrusters, giving the astronaut about 3 m/s of total Δv — enough to fly back to the station if a tether breaks and no more. The earlier **MMU** (Manned Maneuvering Unit, 1984–1985) carried 11 kg of nitrogen and ~25 m/s; Bruce McCandless and Robert Stewart famously flew untethered with it on STS-41B. After Challenger, NASA grounded the MMU as too risky and reverted to tethered EVA with SAFER as backup only.
Four EVA suit families fly today — see /fleet for each: EMU (US Shuttle/ISS, 1981 to present), Orlan-MKS (Russia, latest of the rear-entry family flying since 1977), Feitian (China, Tiangong + Shenzhou), and AxEMU (Axiom, qualifying for Artemis III). The architectural choice that splits them most cleanly is entry geometry — US EMU is front-entry (HUT + LTA assembled), Orlan and Feitian are rear-entry (PLSS hatch on the back) — and operating pressure (4.3 vs 5.7 psi), which decides how much pre-breathe each program requires.
