What Is the NASA ISS EVA Suit?

A.K.A. Extravehicular Mobility Unit

The International Space Station (ISS) serves as a permanent orbital laboratory where humanity pushes the boundaries of science and engineering. While the pressurized modules provide a shirt-sleeve environment for the crew, the exterior maintenance of this colossal structure requires astronauts to venture into the vacuum of space. The machinery that enables this feat is the Extravehicular Mobility Unit (EMU). This system is not merely a garment but a miniature, anthropomorphic spacecraft designed to sustain human life in one of the most hostile environments known. As of late 2025, the EMU is at a pivotal juncture. After more than four decades of service, the fleet is showing signs of advanced age, prompting a complex transition toward a new generation of commercial spacesuits while engineers work tirelessly to maintain the safety and functionality of the current hardware.

Historical Context and Design Philosophy

The EMU was originally conceived in the late 1970s to support the Space Shuttle program. The design philosophy at that time was predicated on the Shuttle’s operational profile: short-duration missions lasting typically two weeks, with the spacesuits returning to Earth after each flight for comprehensive servicing and refurbishment by ground technicians. The suits were robust, modular, and designed to operate primarily in the payload bay of the Shuttle or effectively attached to the robotic arm.

When the International Space Station era began, the operational requirements shifted dramatically. Astronauts were no longer visiting space for short sprints; they were living there for six months to a year. The EMUs were required to remain on orbit for years at a time, with maintenance performed by the crew in microgravity rather than by technicians in a cleanroom on Earth. This shift placed an unforeseen strain on the hardware. The suits currently in use are certified for 25 spacewalks before requiring major ground refurbishment, but the logistical challenge of returning bulky suits to Earth – especially after the retirement of the Space Shuttle in 2011 – forced NASA to develop in-flight maintenance procedures that extend the suit’s life far beyond its original specifications.

The longevity of the EMU is a testament to the engineering prowess of ILC Dover (which manufactures the soft goods) and Collins Aerospace (responsible for the life support systems). However, the hardware is now operating well past its intended design life. Components manufactured in the 1980s are still in circulation, and the system faces obsolescence issues with electronics and sensors that are no longer manufactured.

The Architecture of Survival: System Breakdown

To understand the complexity of maintaining the EMU, it is necessary to dissect its architecture. The suit consists of approximately 18,000 parts and weighs roughly 319 pounds (145 kilograms) when fully loaded on Earth. In the weightlessness of orbit, this mass is manageable, but the inertia remains significant, requiring astronauts to move with deliberate, calculated force.

The Hard Upper Torso (HUT)

The structural foundation of the EMU is the Hard Upper Torso. Unlike the soft fabric suits of the Gemini and Apollo eras, the HUT is a rigid fiberglass shell that acts as a pressure vessel for the upper body. This rigidity provides a stable platform for mounting the life support backpack, the helmet, the display and control module, and the lower torso assembly. The HUT includes the “scye” bearings – complex mechanical joints that allow the shoulder to rotate.

The rigid nature of the HUT creates sizing challenges. Since the shell cannot expand or contract, astronauts must fit into one of the available sizes (Medium, Large, or Extra Large). A proper fit is essential not just for comfort, but for safety. If the shoulder joint of the astronaut does not align perfectly with the scye bearing of the suit, every movement of the arm forces the astronaut’s body against the rigid fiberglass ring. This misalignment can lead to severe shoulder injuries, including rotator cuff tears, which have been a documented occupational hazard for spacewalkers for decades. Inside the HUT, a complex web of water transport tubing and ventilation ducts ensures that cool water and oxygen are distributed evenly across the torso.

The Primary Life Support System (PLSS)

Attached to the back of the HUT is the Primary Life Support System (PLSS). This backpack is the functional heart of the spacesuit. It contains the oxygen tanks, electricity, water cooling equipment, and ventilating fans required to keep the astronaut alive for up to eight hours. The PLSS operates as a closed-loop system.

• Atmosphere Control: The PLSS maintains a pure oxygen atmosphere at a pressure of 4.3 psi (29.6 kPa). This low pressure – roughly one-third of sea-level pressure – is necessary to make the suit flexible. If the suit were pressurized to sea level (14.7 psi), it would be as rigid as a car tire, rendering movement impossible.
• Carbon Dioxide Scrubbing: As the astronaut exhales, carbon dioxide builds up in the loop. The PLSS routes this gas through a contaminant control cartridge. Historically, these used lithium hydroxide (LiOH) to chemically bind with the CO2. Modern iterations use a regenerative Metal Oxide (Metox) system, which can be “baked out” in an oven on the ISS to remove the captured CO2, allowing the canister to be reused multiple times.
• Thermal Regulation: The most complex aspect of the PLSS is thermal control. Without convection or conduction in a vacuum, body heat has nowhere to go. The PLSS uses a sublimator to reject heat. Warmer water from the astronaut’s cooling garment flows into the sublimator, where it passes near a porous metal plate exposed to the vacuum of space. Ice forms in the pores of the plate and then sublimes directly into water vapor, carrying the heat away with it. This process is highly efficient but relies on a consumable supply of feedwater.

The Helmet and Extravehicular Visor Assembly

The helmet is a pressure bubble made of impact-resistant polycarbonate. It attaches to the HUT via a neck ring with a locking mechanism. Unlike science fiction depictions, the helmet does not move with the astronaut’s head; the astronaut looks around by turning their head inside the stationary bubble.

Surrounding the pressure bubble is the Extravehicular Visor Assembly (EVVA). This movable shell provides protection against impact and radiation. It features a gold-coated visor that can be pulled down to reflect intense sunlight. The optical coating is so effective that it acts like a one-way mirror; the astronaut can see out, but observers cannot see in. The EVVA also houses adjustable blinders to block glare, side-mounted floodlights for working during the orbital night, and a high-definition camera (“HelmetCam”) that streams the astronaut’s point of view to Mission Control in Houston.

The Gloves: The Critical Interface

The gloves are the most technically difficult component of the suit to manufacture and the most critical for mission success. They are the only part of the suit that is custom-made for each astronaut. The Phase VI glove currently in use represents the culmination of decades of development.

The glove must perform two contradictory functions: it must be thick enough to prevent a puncture from a sharp edge or a micrometeoroid strike, yet thin enough to allow the astronaut to feel and manipulate small tools. The outer layer of the glove is reinforced with Vectran, a liquid crystal polymer fiber that is five times stronger than steel and resistant to abrasion. The fingertips are covered in room-temperature vulcanizing (RTV) rubber to provide grip.

Inside the glove, a system of straps and pulleys allows the astronaut to adjust the fit on the fly. Despite these advances, working in pressurized gloves is physically punishing. The pressure forces the fingers to extend, meaning the astronaut must constantly exert force to close their hand. This leads to hand fatigue and frequent fingernail injuries, a condition known as onycholysis, where the fingernail separates from the nail bed due to repetitive contact with the rigid glove tips.

Operational Hazards and Safety Protocols

Operating the EMU involves managing a specific set of lethal risks. The vacuum of space offers no forgiveness for mechanical failure or human error.

The Threat of Decompression Sickness

Because the ISS is pressurized to 14.7 psi (sea level) and the EMU operates at 4.3 psi, astronauts are at risk of decompression sickness , commonly known as “the bends.” This occurs when nitrogen dissolved in the bloodstream bubbles out of solution due to the rapid drop in pressure, potentially causing joint pain, paralysis, or death.

To prevent this, NASA employs rigorous “pre-breathe” protocols designed to flush nitrogen from the body before the spacewalk begins. The “In-Suit Light Exercise” (ISLE) protocol is the standard procedure. It involves breathing pure oxygen for hours while performing mild calisthenics to accelerate blood flow and nitrogen off-gassing. An alternative “Campout” protocol involves the spacewalkers spending the night before the EVA in the station’s airlock at a reduced pressure of 10.2 psi, which lowers their nitrogen load and shortens the pre-breathe time required on the day of the spacewalk.

Micrometeoroids and Orbital Debris

The suit acts as a personal armor system against orbital debris . The outermost layer of the EMU is the Thermal Micrometeoroid Garment (TMG). It consists of multiple layers of aluminized Mylar, Neoprene-coated nylon, and Ortho-Fabric (a blend of GORE-TEX, Kevlar, and Nomex). This layup is designed to break up incoming particles and disperse their energy before they can penetrate the pressure bladder. While effective against dust-sized grains moving at hypersonic speeds, the suit cannot protect against larger debris. Ground controllers actively monitor radar tracks of space junk, and a spacewalk can be canceled or aborted if the risk of a conjunction becomes too high.

The Water Intrusion Anomaly

The most persistent and dangerous technical issue facing the current EMU fleet is water intrusion. The cooling system contains water that circulates around the astronaut’s body. If a leak occurs in the PLSS, this water can migrate into the ventilation loop and enter the helmet. In zero gravity, water does not fall; it forms a sticky blob that clings to surfaces.

In July 2013, European Space Agency astronaut Luca Parmitano nearly drowned during a spacewalk when roughly 1.5 liters of water leaked into his helmet. The water covered his eyes, ears, and nose, cutting off his vision and communication. He managed to navigate back to the airlock by memory. The investigation revealed that the leak was caused by silicate buildup clogging the fan pump separator, a component designed to separate water from air.

Since that incident, NASA has implemented several mitigations. These include the “Helmet Absorption Pad” (HAP), a highly absorbent strip placed at the back of the helmet to soak up leaks, and a breathing snorkel that allows the astronaut to draw air from the torso section if the helmet fills with water. Despite these measures, water leaks have continued to plague the aging fleet, leading to several aborted spacewalks and suspended operations in 2024 and 2025.

Comparative Analysis: Global Systems

The EMU is not the only spacesuit operating in low-Earth orbit. The Russian Federation utilizes the Orlan space suit , and the future of American EVA capability lies with the Axiom Space AxEMU. Comparing these systems highlights the different engineering choices made by different space agencies.

The following table details the technical specifications of these three distinct systems.

The Russian Approach: Orlan-MKS

The Russian Orlan-MKS represents a philosophy of robustness and simplicity. Its defining feature is the rear-entry hatch. A cosmonaut can enter the suit without squeezing through a waist ring, drastically simplifying the donning process. The Orlan operates at a higher pressure (5.8 psi), which reduces the pre-breathe time needed to avoid the bends. However, the higher pressure also makes the gloves stiffer, reducing dexterity compared to the EMU. The Orlan is designed for a shorter service life and is typically discarded (pushed away from the station to burn up in the atmosphere) after its operational limit is reached, whereas the EMU was originally designed to be refurbished.

The Next Generation: Axiom AxEMU

The Axiom AxEMU introduces modern materials and manufacturing techniques to the field. It utilizes a rear-entry design similar to the Orlan but incorporates the high dexterity of American glove design. Crucially, the AxEMU is designed for “variable pressure” operations, allowing astronauts to adjust the suit’s internal environment to match different mission phases. It also integrates high-speed data avionics, allowing for better communication and heads-up displays that are not possible with the analog architecture of the current EMU.

The Transition to Commercial Services

The future of the American spacesuit fleet is undergoing a fundamental administrative change. For the entirety of the space program’s history, NASA owned its spacesuits. The agency wrote the specifications, paid contractors to build the hardware, and assumed ownership of every bolt and valve.

Recognizing the high cost and slow pace of this traditional procurement model, NASA initiated the Exploration Extravehicular Activity Services (xEVAS) contract. Under this model, NASA does not buy suits; it purchases “spacewalk services.” The commercial provider retains ownership of the suits and is responsible for their maintenance, logistics, and performance. This shifts the financial risk from the taxpayer to the private sector and encourages competition.

The Departure of Collins Aerospace

Initially, the xEVAS contract was awarded to two providers: Axiom Space and Collins Aerospace. Collins, the legacy provider of the current EMU, was tasked with building the next-generation ISS suit. However, in a significant blow to the program, Collins Aerospace announced in mid-2024 that it would exit the contract. The company cited insurmountable schedule delays and budget overruns, leaving NASA with a single provider for its future EVA needs.

Axiom Space Takes the Lead

With the departure of Collins, Axiom Space became the sole architect of NASA’s future spacewalking capability. Axiom is currently adapting its suit – originally designed for walking on the Moon during the Artemis missions – for the microgravity environment of the ISS. This involves modifying the lower torso to improve flexibility for “floating” rather than walking and ensuring compatibility with the ISS airlock and tool interfaces.

The transition plan is aggressive. The current EMUs are becoming increasingly difficult to maintain, with spare parts dwindling and technical anomalies increasing. NASA aims to have the AxEMU certified and on board the station by 2026 or 2027. This will allow for a crossover period where the old suits are phased out as the new ones come online. The success of this transition is paramount; without functional spacesuits, the ISS cannot be maintained, and the station’s life-extension plans could be jeopardized.

Summary

The NASA ISS EVA spacesuit stands as one of the most successful and enduring pieces of space hardware ever built. It has facilitated the construction of the International Space Station, the repair of the Hubble Space Telescope, and countless scientific experiments. Yet, engineering reality dictates that no machine lasts forever. The recurring cooling loops leaks and the fragility of the supply chain are clear indicators that the EMU has entered its twilight years.

The shift to a commercial services model with the AxEMU represents a necessary evolution. It promises to bring modern technology – including advanced composites, better avionics, and improved mobility – to the spacewalking experience. For the astronauts currently in orbit, the focus remains on rigorous maintenance and adherence to safety protocols, bridging the gap between the legacy hardware of the Shuttle era and the commercial future of human spaceflight. The coming years will be defined by this delicate handoff, ensuring that humanity’s ability to work in the vacuum of space remains uninterrupted.