Walking on the Edge of Everything: The Brutal Science and Hidden Dangers of Spacewalks
On July 16, 2013, Italian astronaut Luca Parmitano was forty-four minutes into his second-ever spacewalk when he noticed something strange: the back of his head felt wet. Within minutes, water was pooling around his ears, creeping toward his eyes, and beginning to flood his nose. Nearly a liter of fluid had invaded his helmet from a malfunctioning water separator in his suit’s cooling system. Unable to smell, taste, or hear clearly, partially blinded, and with no way to wipe his face inside a sealed pressure helmet, Parmitano groped his way hand-over-hand back to the airlock in near-total sensory deprivation. Had he not found the hatch in time, he would have drowned—in space, 250 miles above Earth, traveling at 17,500 miles per hour.
NASA later called it the closest brush with fatality in the history of extravehicular activity, or EVA—the clinical term for what the rest of us call a spacewalk. The incident was a visceral reminder that for all the poetry we attach to the image of a human floating free above our planet, the reality is something far more demanding, more dangerous, and more precisely engineered than it appears. A spacewalk is not an adventure. It is a carefully managed confrontation with an environment that wants to kill you in at least a dozen different ways simultaneously.
The Architecture of Preparation: Months Before a Single Bolt Is Turned
The first thing to understand about spacewalks is that almost nothing about them is spontaneous. A single EVA on the International Space Station typically requires four to eight hours of preparation for every hour of work performed outside. For a planned six-hour excursion, the groundwork can span months.
Mission planners at NASA’s Johnson Space Center in Houston begin by defining the objective—whether that’s replacing a faulty gyroscope, installing solar array upgrades, or retrieving science experiments from the station’s exterior. Once the task is scoped, engineers build a detailed timeline called a “cuff checklist,” a laminated card worn on the astronaut’s wrist that breaks down every action into discrete steps, sometimes with time allocations as granular as two minutes. These checklists are tested and refined through dozens of rehearsals in the Neutral Buoyancy Laboratory (NBL) in Houston, a six-million-gallon pool that contains full-scale mock-ups of ISS modules.
Astronauts scheduled for EVAs spend hundreds of hours underwater. The NBL doesn’t simulate weightlessness perfectly—neutral buoyancy creates drag that genuine microgravity doesn’t—but it’s close enough to train muscle memory for working in pressurized suits with limited dexterity. Every tool is tethered. Every procedure is rehearsed. “We practice until we can’t get it wrong,” NASA astronaut Peggy Whitson, who performed ten career spacewalks—the American record—has noted in interviews. “And then we practice some more.”
The suit itself demands its own preparation protocol. The EMU, or Extravehicular Mobility Unit, is essentially a one-person spacecraft. It weighs 280 pounds on Earth (though effectively nothing in orbit) and consists of fourteen layers of material, including an inner cooling garment threaded with water tubes, a pressure bladder, multiple insulating layers, and an outer shell of Ortho-Fabric designed to resist micrometeoroids and temperature extremes ranging from minus 250 to plus 250 degrees Fahrenheit. Suiting up takes roughly forty-five minutes and requires assistance.
Before the crew even reaches the airlock, they must spend several hours breathing pure oxygen to purge nitrogen from their bloodstreams. This pre-breathe protocol prevents decompression sickness—the same condition that afflicts divers who surface too quickly—because the EVA suit operates at a lower pressure (4.3 psi) than the station’s interior (14.7 psi). Skip this step, and nitrogen bubbles could form in the astronaut’s joints and bloodstream, causing excruciating pain or worse.
The Physics of the Void: What Astronauts Are Actually Working Against
Once outside, astronauts encounter an environment that defies almost every physical intuition developed over a lifetime on Earth. There is no up or down. There is no ambient sound. Temperature swings of 500 degrees Fahrenheit can occur within minutes as the station cycles between sunlit and shadowed portions of its 92-minute orbit. And there is the vacuum itself—a permanent, patient adversary.
Without the suit’s pressurization, a human being exposed to the vacuum of space would lose consciousness in about 15 seconds as oxygen in the blood depletes. Fluids in soft tissue would begin to vaporize. The process is not the explosive decompression of Hollywood films; it is quieter and faster and in some ways more sinister. This knowledge informs every decision made during an EVA.
Mobility is its own challenge. The pressurized suit resists movement. Gripping, twisting, and manipulating tools requires significantly more physical force than the same actions on Earth, which is why spacewalkers often develop hand fatigue within hours and why tools are specifically designed for use with pressurized gloves. The gloves are among the most sophisticated components of the suit and also among the most vulnerable: even small abrasions can compromise their integrity. NASA tracks glove wear rigorously, with suits returned to Earth after EVAs for detailed inspection.
Navigation and translation—the term used for moving from point to point on the station’s exterior—requires constant attention to tethering. Astronauts are always clipped to safety tethers, and the practice of attaching before detaching is drilled until it is reflexive. The ISS travels at nearly 5 miles per second. An unattached astronaut who drifted even a few feet from the station would be essentially unreachable. The SAFER (Simplified Aid for EVA Rescue) jetpack, worn on the back of the suit, provides a last-resort means of propulsion in such a scenario, but it carries only enough propellant for a single rescue attempt. It has never been used in an actual emergency—a record everyone in the program is highly motivated to preserve.
Inside Mission Control: The Third Astronaut You Never See
Every spacewalk has two people outside, but there is effectively a third astronaut critical to its success: the IV, or “intravehicular” crew member who remains inside the station and serves as the primary communicator, document reader, and emergency coordinator. Meanwhile, dozens of specialists in Houston monitor every parameter in real time.
Flight controllers track suit pressure, oxygen levels, carbon dioxide readings, battery charge, and the position of each astronaut relative to the station’s structure. They watch for “red lines”—threshold values that automatically trigger abort protocols. If suit pressure drops below a certain level, the EVA ends immediately, regardless of where the astronauts are on the station’s exterior. If carbon dioxide readings rise—a sign that the suit’s scrubbing system is failing—the crew heads back.
Communication is constant but disciplined. The capcom, or capsule communicator, serves as the primary voice between Houston and the crew, and conversation follows tight protocols to minimize confusion. During complex tasks, controllers can call a “hold”—pausing all activity to assess a problem—without the crew needing to improvise solutions in a high-pressure environment.
This layered system of oversight reflects decades of accumulated lessons. The 2013 Parmitano incident, for example, revealed that flight controllers had not acted quickly enough when early warning signs appeared during his first spacewalk weeks earlier, when a small amount of moisture had been noted but attributed to a leaking drink bag. The investigation that followed resulted in new monitoring procedures and faster abort triggers for helmet moisture detection.
A Catalogue of Things That Can Go Wrong
The Parmitano near-drowning is the most dramatic recent near-miss, but the history of EVAs is populated with incidents that illuminate just how narrow the margins are.
In 1965, on the first American spacewalk, Ed White had such difficulty getting back inside Gemini IV that he and command pilot Jim McDivitt both had to muscle the hatch closed against the pressurized suit. It was a reminder that even reentry—the “safe” part of an EVA—has its hazards.
In 2012, NASA astronaut Sunita Williams and Japan Aerospace Exploration Agency (JAXA) astronaut Akihiko Hoshide spent 8 hours and 17 minutes on a spacewalk to replace an electrical power unit—an assignment expected to take far less time. A jammed bolt, corroded by years of exposure to the space environment, refused to seat correctly. The pair improvised, using a spare cleaning brush to remove debris, and ultimately succeeded, but the incident highlighted how even small anomalies become major problems when they occur in space.
Micrometeoroid and orbital debris impacts represent a persistent, probabilistic threat. The ISS is hit by tiny particles regularly; most are so small that the station’s Whipple shields—layers of spaced aluminum and Kevlar—absorb the impact. But a particle traveling at orbital velocity of roughly 17,500 mph carries kinetic energy far disproportionate to its size. In 2021, the ISS’s robotic arm, Canadarm2, was struck by debris that left a hole 5 millimeters in diameter in one of its boom segments. Had that particle struck a suit during an EVA, the consequences could have been catastrophic. NASA estimates there are roughly 27,000 trackable pieces of debris in orbit; the number of smaller, untrackable fragments runs into the millions.
Thermal management is a constant concern. The suit’s Active Thermal Control System circulates water to absorb metabolic heat generated by the astronaut’s body during physical exertion. If that system degrades, the astronaut can overheat quickly—a threat that is especially acute during strenuous repair work. Conversely, periods of reduced activity in shadow can risk hypothermia if the suit’s heating elements underperform.
The Evolution of the Spacesuit: Why the EMU Is Both Marvel and Liability
The EMU currently used for ISS EVAs was designed in the 1970s and has been continuously upgraded since. That longevity is testimony to its fundamental soundness—but it also means that America’s primary spacewalk suit is, by any objective measure, old. NASA has been developing the next-generation Exploration Extravehicular Mobility Unit (xEMU) for years, with a design intended to be more flexible, more capable, and better suited for lunar and Martian surface operations. The suit was central to NASA’s Artemis lunar program plans, though development has faced delays and budget challenges.
Critics within the aerospace community have noted that the current suit inventory is aging and that the number of suits available for ISS operations has declined due to components failing and limited resupply options. A 2023 report by NASA’s Office of Inspector General warned that the agency faces a “significant risk” of not having enough functioning EMUs to support ISS operations through the station’s planned decommission date in the early 2030s. The report identified aging hardware, manufacturing gaps, and supply chain issues as compounding factors.
The commercial sector has entered this space. Axiom Space holds a contract with NASA to develop a new spacesuit for Artemis lunar surface operations and potential ISS use. Collins Aerospace is working on a competing design. Whether these efforts produce operational hardware on schedule remains to be seen, but the trajectory suggests that the next decade of EVA will be defined as much by industrial and procurement challenges as by the physics of the void.
The Future of Spacewalking: More EVAs, Higher Stakes
The ISS has hosted more than 270 spacewalks since its first modules were connected in 1998, accumulating over 1,700 hours of extravehicular activity. That number will grow as the station continues to require maintenance and upgrades to sustain operations. China’s Tiangong space station has been conducting its own EVA program with increasing frequency as the Chinese space agency expands its orbital capabilities and trains its taikonauts in techniques that parallel, and in some cases deliberately diverge from, NASA protocols.
Looking further ahead, the return to the Moon under Artemis and the long-term ambitions for Mars will demand spacewalking capabilities that far exceed anything the current EMU can provide. Lunar surface EVAs will involve walking rather than floating, in partial gravity with abrasive regolith that can contaminate seals and degrade suit performance. Mars EVAs—if they happen within this century—will add radiation exposure, communication delays of up to 24 minutes each way (eliminating real-time Mission Control support), and extreme dust to the equation.
These challenges have prompted some researchers to explore alternative approaches. Mechanically counterpressure suits—which use tight-fitting mechanical garments instead of gas pressurization to protect the body—have been studied for decades as a potentially more flexible, mobile option. They remain experimental but illustrate that the gas-pressurized suit architecture may not be the final word in human protection in space.
What the history of spacewalks ultimately demonstrates is not that space exploration is too dangerous to pursue, but that the acceptable margin of risk in space is razor-thin, and that closing that margin requires obsessive attention to detail, relentless practice, and an institutional culture that treats near-misses as invaluable data rather than lucky escapes. Luca Parmitano made it back inside. The lesson NASA drew was not relief—it was a systematic audit of everything that had allowed the situation to develop in the first place.
That is the real discipline of the spacewalk: not courage, though courage is required, but the sustained, collective intelligence of thousands of people working to compress the distance between what can go wrong and what actually does.