Extravehicular Activity (EVA) refers to any task performed by an astronaut or cosmonaut outside a spacecraft, often called a “spacewalk” in Earth orbit or a “moonwalk” on the lunar surface. These activities are fundamental to space exploration, allowing humans to directly interact with their environment beyond a vehicle’s confines. From maintaining space stations to deploying scientific instruments, EVAs play a significant role in expanding our presence in space.
The concept of venturing outside a spacecraft became a reality in the mid-20th century. Soviet cosmonaut Alexei Leonov performed the first spacewalk on March 18, 1965, exiting his Voskhod 2 capsule. A few months later, on June 3, 1965, American astronaut Ed White conducted the first U.S. spacewalk during the Gemini 4 mission. These events demonstrated the feasibility of working in the vacuum of space, paving the way for future complex missions.
The Hostile Environment and Spacesuits
Space presents a harsh environment for any unshielded human, characterized by a near-perfect vacuum that would cause bodily fluids to boil. Temperatures fluctuate dramatically, ranging from approximately -250°F (-157°C) in shadow to over +250°F (+121°C) in direct sunlight. This extreme thermal variation demands robust protection. The environment also exposes individuals to harmful solar and cosmic radiation, along with the threat of impacts from tiny, high-velocity micrometeoroids and orbital debris.
To counter these dangers, astronauts wear specialized spacesuits, such as the Extravehicular Mobility Unit (EMU) used by NASA. These suits function as personalized miniature spacecraft, providing a pressurized, breathable atmosphere, typically 4.3 pounds per square inch of pure oxygen. An intricate liquid cooling and ventilation garment worn underneath circulates water to regulate the astronaut’s body temperature. Layers of fabric and insulation within the suit offer protection against thermal extremes, while robust outer layers shield against radiation and potential micrometeoroid strikes.
Spacesuits also contain self-contained life support systems, including oxygen tanks, carbon dioxide removal systems, and power sources. These systems are housed in a backpack-like Portable Life Support System (PLSS) that attaches to the suit. The suit’s design allows for mobility through flexible joints, enabling astronauts to perform tasks in a pressurized environment. This comprehensive protection allows humans to survive and work productively outside the spacecraft.
Extensive Training and Operational Procedures
Astronauts undergo extensive training to prepare for the complexities of an EVA. Much of this preparation takes place in facilities like the Neutral Buoyancy Laboratory (NBL) at NASA’s Johnson Space Center in Houston, Texas. The NBL is a large indoor pool designed to simulate the microgravity environment of space. Astronauts practice their spacewalk tasks underwater, wearing their full spacesuits, weighted to achieve neutral buoyancy.
These underwater simulations are performed for hundreds of hours, allowing astronauts to develop muscle memory and perfect intricate procedures for various maintenance, assembly, or repair tasks. Practicing in a simulated environment helps them become familiar with the tools and equipment they will use and understand how their bodies will move in a near-weightless state. Crew members also rehearse communication protocols and emergency procedures.
An EVA follows a meticulously planned sequence of operational phases. The pre-EVA phase involves several hours of “pre-breathe” protocols, where astronauts breathe pure oxygen to purge nitrogen from their bloodstream, preventing decompression sickness (the “bends”) when transitioning to the lower pressure inside the suit. During the EVA, astronauts execute their planned tasks, constantly communicating with mission control. Post-EVA procedures include doffing the suit, re-pressurizing the airlock, and a thorough debriefing.
Purposes and Tools of a Spacewalk
Extravehicular activities serve many purposes, supporting long-duration space missions and scientific endeavors. A frequent reason for EVAs is the maintenance and repair of spacecraft and space stations. Astronauts perform tasks like replacing aging components, fixing external damage, or upgrading systems that cannot be serviced from inside. These activities keep complex orbital outposts like the International Space Station (ISS) operational and safe.
Another application of EVAs is the assembly and construction of large space structures. The ISS, for example, was largely built piece by piece through a series of spacewalks. Astronauts connect modules, install solar array segments, and attach external hardware, effectively building a complex orbital facility by hand. This hands-on construction capability is currently unmatched by robotic systems for large-scale projects.
EVAs are also performed for the deployment and retrieval of scientific instruments or small satellites. Astronauts can precisely position sensitive equipment on the exterior of a spacecraft or recover payloads that have completed their mission. Spacewalks also allow for scientific experiments that require direct exposure to the space environment, such as material science tests or biological studies.
Astronauts utilize a range of specialized equipment and tools to accomplish their tasks during an EVA. Tethers secure them to the spacecraft and prevent accidental drift. Specialized wrenches, power tools, and custom-designed clamps are used for assembly and repair. The Mini Work Station, a portable platform attached to the suit, provides a convenient place to store and organize tools. For safety, astronauts wear the SAFER (Simplified Aid for EVA Rescue) jetpack, a small propulsion unit that can be used to return to the spacecraft if they become untethered.
Risks and Notable Achievements
Despite meticulous planning and extensive training, EVAs inherently carry significant risks. Equipment failure, such as a suit malfunction or loss of power, can quickly become life-threatening. The threat of micrometeoroid strikes or impacts from orbital debris remains a concern, as even a tiny particle can cause damage to the suit or spacecraft at orbital velocities. Human error, fatigue, or misjudgment can also lead to complications during complex tasks. The physical demands of working in a pressurized suit and exposure to radiation also take a physiological toll on the astronaut’s body.
Despite these challenges, EVAs have led to numerous milestones and records in space exploration. Astronauts have performed spacewalks lasting over eight hours, demonstrating endurance. The cumulative time spent outside spacecraft has reached thousands of hours across hundreds of individual EVAs. These include complex repair missions, such as the servicing of the Hubble Space Telescope, which involved multiple spacewalks to upgrade and repair the observatory. Such achievements underscore the human capacity for ingenuity and perseverance in the demanding environment of space.