EVA stands for extravehicular activity, the formal term for any work an astronaut performs outside the pressurized environment of a spacecraft or space station. In everyday language, it is called a spacewalk, though the term applies equally to surface excursions on the Moon or, eventually, Mars. What sounds simple in a sentence is one of the most physically punishing and operationally complex things a human being can do, requiring a self-contained spacesuit that functions as a miniature spacecraft wrapped around one person’s body.
What the Term Actually Covers
The phrase “extravehicular activity” was coined during the early days of crewed spaceflight to describe any task requiring a crew member to leave the pressurized cabin. EVA spacesuits must provide pressurization, life support including breathable oxygen and carbon dioxide removal, thermal regulation, and protection from radiation and orbital debris, all while remaining light and flexible enough for the astronaut to do useful work.1Springer. Extravehicular Activity (EVA) The first EVA was performed by Soviet cosmonaut Alexei Leonov in 1965, when he floated outside his Voskhod 2 capsule for roughly twelve minutes. Since then, the definition has expanded to include everything from repairing the Hubble Space Telescope in orbit to collecting rock samples on the lunar surface.
The word “activity” does a lot of quiet work in the acronym. An EVA is not a casual stroll. Every minute outside the spacecraft is planned, scripted, and rehearsed, with a ground-control team and an intravehicular (IV) crew member inside the station coordinating each step. EVAs on the International Space Station typically last six to eight hours, and astronauts often describe them as the most physically exhausting days of their careers.
How a Spacesuit Keeps You Alive
A spacesuit during EVA is essentially a pressurized bubble with arms and legs. Inside that bubble, a portable life support system handles oxygen supply, carbon dioxide scrubbing, temperature control, and humidity management. The suit operates at a lower pressure than the spacecraft cabin, which makes the joints flexible enough to move but also creates a risk of decompression sickness, a problem discussed further below.
Thermal control is one of the trickiest engineering challenges. In orbit, temperatures can swing from extreme cold in shadow to scorching heat in direct sunlight within minutes. The primary thermal management tool is a liquid cooling and ventilation garment worn against the skin, essentially a network of thin water-filled tubes that absorbs body heat and routes it to a heat exchanger. A thermal control valve regulates how much coolant flows through the garment to keep the astronaut comfortable.252nd International Conference on Environmental Systems. Space Suit Portable Life Support System Thermal Control Valve Ball Design
Simulations of an eight-hour EVA suggest the suit consumes roughly one kilogram of lithium hydroxide for COâ‚‚ scrubbing and about 2.7 kilograms of water for cooling under standard working conditions. The harder the astronaut works, the faster those consumables are used up: tripling the metabolic heat output increases lithium hydroxide consumption by about two and a half times.3Advances in Mechanical Engineering. Joint modeling and dynamic analysis of the microenvironment and life support performance of extravehicular spacesuits That relationship between exertion and resource use is one reason EVA timelines are so carefully choreographed. A crew member who rushes a bolt or fights a stuck connector burns through suit resources faster than planned.
The Physical Toll of Working in a Suit
The pressurized gloves are the biggest culprit when it comes to physical strain. Imagine trying to grip tools while wearing inflated rubber gloves that constantly push your fingers open. Over a six-to-eight-hour EVA, the repeated effort to close your hands against that internal pressure can cause fingernail damage, bruised knuckles, and overuse injuries in the forearms and shoulders. The demands on the upper extremity from suit constraints, zero-gravity dynamics, and temperature extremes combine to create conditions ripe for repetitive strain injuries.4Advances in Mechanical Engineering. Physical demands and injuries to the upper extremity associated with the space program
Astronauts often lose fingernails during intensive EVA training and missions. The problem is so well known that some astronauts have preventively had their fingernails removed before long-duration stays. Hand fatigue tends to accumulate across consecutive EVA days, which is why mission planners usually build in rest days between outings. Despite decades of suit redesign, hand mobility remains one of the most stubborn engineering problems in EVA hardware.
Decompression Sickness and the Prebreathe Problem
Because the suit operates at lower pressure than the cabin, nitrogen dissolved in the astronaut’s blood can form bubbles during the transition, the same mechanism behind “the bends” in scuba diving. To prevent this, astronauts breathe pure oxygen before an EVA to flush nitrogen from their tissues. On the ISS, the current prebreathe protocol can take hours and involves staged pressure reductions in the airlock.
The ISS airlock itself is divided into two sections: an equipment lock where suits are maintained and prepped, and a crewlock that is depressurized to allow the suited crew members to exit the station.5SAE International. International Space Station (ISS) Airlock Crewlock Depressurization Methods The process is time-consuming but necessary, and it represents one of the hidden costs of EVA: every hour spent prebreathing is an hour not spent doing work outside.
NASA has been exploring ways to shorten this process for future missions. One approach involves keeping the habitat at a lower overall pressure with a higher oxygen percentage than the ISS uses. Under an exploration atmosphere of about 56.5 kPa with 34 percent oxygen, modeling suggests a prebreathe as short as 15 minutes could be feasible, though estimated decompression sickness risk for various EVA scenarios under that protocol still ranges between roughly 6 and 12 percent.6Acta Astronautica. Modeling a 15-min extravehicular activity prebreathe protocol using NASA’s exploration atmosphere (56.5 kPa/34% O2) That is a nontrivial risk, and fine-tuning the balance between crew time savings and decompression safety is an active area of research.
How Astronauts Train for EVAs
The primary training tool for spacewalks is the Neutral Buoyancy Laboratory (NBL), a massive pool containing a full-scale mockup of ISS modules. Underwater training dates back to the mid-1960s, when mission planners realized that parabolic “vomit comet” flights, which offer only about 30 seconds of weightlessness per parabola, were nowhere near long enough to practice complex EVA tasks. The lesson came directly from Gemini-era experience, after astronaut Gene Cernan struggled with EVA procedures on orbit despite completing parabolic flight training.7Acta Astronautica. 21st Century extravehicular activities: Synergizing past and present training methods for future spacewalking success
Neutral buoyancy is not a perfect simulation of microgravity. Water creates drag that does not exist in space, and the weight of the suit is distributed differently when submerged than when floating freely. But it allows astronauts to rehearse tasks for hours at a time, building muscle memory for movements like turning bolts, connecting fluid lines, and translating along handrails. Every ISS EVA is practiced repeatedly in the pool before it is attempted on orbit, and the ratio of training hours to actual EVA hours is substantial.
Virtual reality has recently entered the training picture as well. Researchers have begun using VR to simulate surface EVAs for lunar and Martian missions, where terrain, visibility, and task complexity differ sharply from orbital spacewalks. These simulations help evaluate not just physical readiness but cognitive performance under varied workloads.
Cognitive Demands You Might Not Expect
EVA is usually discussed in physical terms, but the cognitive load is just as real and arguably harder to manage. An astronaut on a surface EVA has to make continuous decisions that balance safety, time constraints, and mission objectives simultaneously. Situational awareness is the linchpin: knowing where you are, what your suit status is, what the terrain looks like, and whether anything is going wrong. Factors that sharpen awareness include adequate ground support, clear visuals of the environment, and manageable physical exertion. Factors that erode it include fatigue from poor sleep, visual obstructions like harsh shadows or bright sunlight, and high physical workload such as pushing heavy equipment uphill.8npj Microgravity. Identifying cognitive capabilities required for optimal surface extravehicular activity performance
The physical-cognitive interaction is worth highlighting. High physical exertion during EVA does not just tire the body; it independently increases cognitive workload, meaning it gets harder to think clearly even about tasks that are not physically demanding. In simulated surface EVAs using virtual reality, experimentally increasing the difficulty of geological sample identification led to measurably higher subjective cognitive workload, reduced cognitive performance, altered physiological responses, and worse performance on key EVA tasks.9PubMed Central. Characterizing cognitive workload during simulated surface extravehicular activity with integrated virtual reality This is a practical concern for future lunar and Mars EVAs, where astronauts will need to make real-time geological judgments while physically wrestling with suits, tools, and terrain. The risk is not just making bad decisions but not recognizing that you are making them.
Walking on the Moon Is Not Like Walking on Earth
Apollo footage of astronauts hopping and bounding across the lunar surface was not just playfulness; it reflected a genuine change in how humans move in low gravity while wearing a pressurized suit. On Earth, people naturally transition from walking to running at a predictable speed. On the Moon, with only one-sixth Earth’s gravity and a suit that weighs about as much as the person inside it, that transition happens at a very different point.
Research analyzing Apollo footage found that the walk-run transition speed in suited lunar conditions was significantly lower than what would be predicted for an unsuited person in the same gravity. A substantial portion of that shift, roughly 60 percent of the difference, appears attributable to the suit’s self-support: the suit is so stiff and massive relative to the wearer that it changes the body’s natural locomotion patterns.10PubMed Central. The Apollo Number: Space Suits, Self-Support, and the Walk-Run Transition Astronauts ended up adopting a distinctive loping gait or a skip not because it was fun but because it was biomechanically efficient given the constraints.
Studies using NASA’s Active Response Gravity Offload System, which simulates reduced gravity by partially suspending the body, have confirmed that fundamental changes occur in muscle activation patterns under simulated Martian and lunar gravity. Significant shifts were measured in lower-leg muscle groups during walking, running, and skipping under both 0.38g (Mars) and 0.17g (Moon) conditions.11Acta Astronautica. Effects of walking, running, and skipping under simulated reduced gravity using the NASA Active Response Gravity Offload System (ARGOS) These findings feed directly into suit and boot design for upcoming missions. If the suit forces the body to move in unnatural ways, it wastes energy and increases injury risk over multi-hour surface excursions.
The Lunar Dust Problem
Apollo astronauts quickly discovered that lunar dust, or regolith, is one of the most insidious hazards of surface EVAs. The particles are fine, sharp-edged, electrostatically charged, and cling to everything. They abraded suit fabrics, jammed zippers, and reduced joint mobility over just a few days. For short Apollo missions, this was tolerable. For weeks-long stays planned under the Artemis program, it is a serious engineering problem.
Conventional dust mitigation methods used during Apollo, like brushing and vacuuming, actually made things worse by grinding particles into suit fabrics. Researchers have been testing liquid nitrogen sprays as an alternative, and the results are promising: in vacuum conditions simulating an airlock, the sprays removed an average of over 98 percent of dust mass, including about 96 percent of the most dangerous fine particles below 10 micrometers. When suit fabric samples were cycled through repeated spray washings, the material degradation was minimal compared to traditional brushing.12Acta Astronautica. Lunar dust removal and material degradation from liquid nitrogen sprays
The suit materials themselves are also being reconsidered. Reviews of advanced materials for future lunar EVA suits have focused on the external layers that take the most abuse from regolith abrasion and space radiation, evaluating candidates based on mechanical durability and resistance to the harsh vacuum and thermal environment.13Advanced Materials Technologies. Advanced Materials for Future Lunar Extravehicular Activity Space Suit Another line of research has explored carbon nanotube coatings that could make suit fabrics inherently dust-repellent, with experiments showing the ability to repel more than 80 percent of lunar simulant particles.14Acta Astronautica. Self-cleaning spacesuits for future planetary missions using carbon nanotube technology Whether any of these solutions survives the transition from laboratory to actual lunar operations remains to be seen, but the consensus is clear that Apollo-era approaches will not be good enough.
American Suits, Russian Suits, and the Commercial Future
The United States and Russia have taken different design philosophies to EVA suits for decades. The American Extravehicular Mobility Unit (EMU) used on the Space Shuttle and ISS is modular, with interchangeable pieces sized to fit different astronauts. The Russian Orlan suit, used on Mir and later the ISS, is a rear-entry design where the astronaut climbs in through a hatch on the back, which allows faster donning and a more standardized fit. Both approaches reflect different priorities in task requirements, host vehicle design, and life support architecture.15SAE International. Extravehicular Individual Life Support: A Comparison of American and Russian Systems
The next chapter in EVA suit development is being partly written by private companies. Final Frontier Design, for example, has developed and begun beta testing a commercially focused EVA suit system, covering everything from the pressure garment and bearings to the liquid cooling garment and life support system.16New Space. A Commercial Extravehicular Activity Space Suit Axiom Space holds NASA’s contract to provide suits for the Artemis lunar surface missions, and SpaceX has demonstrated its own EVA-capable suit during the Polaris Dawn mission. This shift toward commercial providers mirrors what happened with launch vehicles over the past decade and could drive down cost while accelerating innovation, though the safety bar for a garment that keeps someone alive in vacuum is understandably high.
The design priorities for these next-generation suits are shaped by where they will be used. An orbital EVA suit optimizes for upper-body mobility because the astronaut’s legs mostly just dangle. A lunar surface suit needs to prioritize walking, bending, kneeling, and picking up objects. Engineers are also grappling with sizing: the EMU’s limited sizing options famously contributed to a planned all-female spacewalk being postponed in 2019 because a second medium-sized torso was not available on station. Future suit programs have pledged to accommodate a much wider range of body sizes, though translating that pledge into hardware that fits well, moves freely, and withstands the brutality of the space environment is one of the hardest open problems in human spaceflight.