Extravehicular activity, commonly known as EVA or spacewalking, is any activity performed by an astronaut outside the protective environment of a spacecraft while in the vacuum of space or on the surface of another celestial body. The te…
An astronaut's spacesuit functions as a miniature spacecraft wrapped around the human body. The Primary Life Support Subsystem (PLSS), worn as a backpack, contains high-pressure oxygen tanks that release breathable air into the suit at carefully controlled rates. This system maintains pressure at about 4.3 psi, roughly one-third of sea-level atmospheric pressure but sufficient to keep blood from boiling and lungs functioning.
As the astronaut breathes, they exhale carbon dioxide that would quickly become toxic in the sealed environment. Lithium hydroxide canisters chemically scrub CO2 from the air, while fans circulate the atmosphere to prevent pockets of stale air from forming around the astronaut's face. The system also regulates temperature through a network of cooling water tubes woven into an undergarment, since the vacuum of space provides no air for natural heat dissipation.
Modern suits carry enough consumables for seven to eight hours of EVA work, with additional reserves for emergencies. Sensors continuously monitor oxygen levels, CO2 concentration, pressure, and temperature, relaying warnings to both the astronaut and ground controllers if any parameter approaches dangerous limits.
The airlock serves as a decompression chamber between the spacecraft's normal Earth-like atmosphere and the vacuum beyond. Before an EVA begins, astronauts enter this sealed compartment and close the inner hatch, isolating themselves from the main cabin. The airlock then slowly vents air into space over 30 to 60 minutes, reducing pressure from the spacecraft's 14.7 psi to the suit's operating pressure of 4.3 psi.
This gradual depressurization prevents the bends, the same nitrogen bubble formation in blood that afflicts scuba divers who surface too quickly. Astronauts typically pre-breathe pure oxygen for several hours before EVA to flush nitrogen from their bloodstream, further reducing this risk. Some modern suit designs operate at higher pressures to minimize pre-breathe time, though this creates challenges for joint mobility.
When astronauts return from spacewalking, the process reverses. They enter the airlock from outside, seal the outer hatch, and the chamber slowly repressurizes with air from the spacecraft's reserves. Only after pressure equalizes can they safely open the inner hatch and remove their helmets, rejoining their crewmates in the shirtsleeve environment.
During EVA, astronauts clip retractable safety tethers to anchor points on the spacecraft's exterior, creating a physical lifeline back to safety. These tethers, typically 85 feet long and made of braided stainless steel cables, can support thousands of pounds of force. As astronauts move along the hull, they continuously detach and reattach these lines in a process called "tether translation," always maintaining at least one connection.
The consequences of becoming untethered are severe. Without a connection, even a gentle push against the spacecraft would send an astronaut drifting away at constant velocity, with no air resistance to slow them down. While the Simplified Aid for EVA Rescue (SAFER) jetpack provides a backup propulsion system with 24 small nitrogen thrusters, its fuel supply allows only about 10 feet per second of velocity change—enough for minor corrections but not for rescue from serious drift.
Engineers color-code and label tether attachment points along planned EVA routes, creating a "road map" on the station's exterior. Astronauts practice these routes extensively in underwater training facilities, developing muscle memory for the attachment sequence. The International Space Station features over 160 handrails and tether points strategically positioned for the most common maintenance and assembly tasks.
In microgravity, every action produces an equal and opposite reaction with no ground to absorb excess force. When an astronaut pushes against the spacecraft to move, they must grab a handrail or foothold simultaneously to prevent rotating uncontrollably. The spacesuit itself weighs nothing in orbit but retains its full mass, creating significant inertia that resists changes in motion—stopping or turning requires the same muscular effort as moving a 300-pound object on Earth.
Foot restraints provide crucial stability for tasks requiring two free hands. These devices, resembling ski bindings, lock boots onto the spacecraft's surface, allowing astronauts to pivot and lean while keeping their workspace stable. For translation over longer distances, astronauts pull themselves hand-over-hand along handrails, moving in a motion similar to crossing monkey bars but with their entire body mass floating behind them.
The bulky pressurized suit severely limits flexibility and dexterity. Glove fingers resist bending against internal pressure, making even simple grips exhausting after hours of work. Astronauts frequently report hand fatigue as the most physically demanding aspect of EVA. The helmet restricts peripheral vision and prevents astronauts from seeing their own feet, forcing them to work largely by feel and remembered positions of tools and attachment points.
Standard Earth tools are nearly useless in spacesuits, where pressurized gloves eliminate tactile feedback and fine motor control. EVA tools feature oversized handles, textured grips, and mechanisms operable with crude pinching motions. Many incorporate ratcheting systems that require only gross hand squeezing rather than precise finger work. Pistol-grip tools, like the battery-powered drill used for station assembly, allow astronauts to operate triggers while their hands fight against glove pressure.
Every tool tethers to the astronaut's suit or worksite, since a dropped bolt or wrench becomes a permanent satellite orbiting Earth at 17,500 mph. Specialized tool bags clip to the suit, with each implement secured by retractable cords. Some tasks use "torque multipliers"—lever-based tools that provide the mechanical advantage to loosen bolts that may have cold-welded together in the temperature extremes of space, which swing from 250°F in sunlight to -250°F in shadow.
Robotic assistance increasingly supports complex repairs. The Canadarm2 robotic arm can transport astronauts to worksites across the station's 357-foot length, functioning as a mobile work platform. For Hubble Space Telescope servicing missions, astronauts used specialized tools designed specifically for each repair task, including one that cut through warped handrails and another that installed 116 tiny screws into new electronics panels—operations practiced hundreds of times underwater before execution in orbit.