Pillar Page guide : Astronauts, Training, and Life in Orbit
Introduction
Imagine training for the most dangerous part of your job by spending six hours underwater in a spacesuit that weighs 300 pounds on land. This is the reality for astronauts preparing for spacewalks—or EVAs (extravehicular activities) as NASA calls them. The Neutral Buoyancy Laboratory in Houston, Texas, houses the world’s largest indoor pool, where astronauts rehearse every movement they’ll make while floating outside the International Space Station.
A single six-hour spacewalk requires approximately 40 hours of underwater training. Astronauts practice in NASA’s 6.2 million-gallon pool, where they work on full-scale mockups of ISS modules while support divers ensure safety and adjust their buoyancy to simulate weightlessness. This intensive preparation is essential—in the vacuum of space, there are no second chances, no timeouts, and mistakes can be fatal.
The Neutral Buoyancy Laboratory: NASA’s Underwater Space Station
The Neutral Buoyancy Laboratory (NBL) is an impressive facility—a pool 202 feet long, 102 feet wide, and 40 feet deep, containing 6.2 million gallons of water. Located at the Sonny Carter Training Facility near NASA’s Johnson Space Center in Houston, this enormous pool can accommodate full-scale mockups of ISS modules, the Space Shuttle payload bay, and various spacecraft components.
The term ‘neutral buoyancy’ refers to the condition where an object neither sinks nor floats but remains suspended at a constant depth. By carefully adding or removing weights from the spacesuit, technicians can adjust an astronaut’s buoyancy until they hover motionless in the water—the closest simulation of weightlessness available on Earth.
Inside the pool, astronauts wear modified versions of the Extravehicular Mobility Unit (EMU)—the white spacesuit used for spacewalks. These training suits contain the same gloves, helmet, and life support backpack as flight suits, but they’re weighted to achieve neutral buoyancy rather than provide pressure protection. The suits weigh approximately 300 pounds on land due to all the added weights, making it impossible for astronauts to walk to the pool—they’re lowered in by crane.
Why Underwater Training Works
While neutral buoyancy isn’t perfect weightlessness—water resistance affects movement differently than the vacuum of space—it’s the best Earth-based simulation available. Astronauts can practice the same body positions, hand movements, and tool manipulations they’ll use in orbit. The water’s resistance actually helps in one way: it slows movements down, giving astronauts time to think and react, much like the deliberate pace required in actual spacewalks.
The pool environment allows for repeated practice of critical tasks. If an astronaut makes a mistake during training—dropping a tool, positioning incorrectly, or taking too long on a procedure—they can immediately try again. This repetition builds muscle memory that becomes automatic during the stress of actual EVAs.
Perhaps most importantly, underwater training reveals problems before they become crises in space. If a procedure proves too difficult, requires too much time, or poses unexpected challenges, engineers can redesign tools, modify procedures, or add additional crew members to the task—all based on NBL training sessions.
A Typical Training Session
A standard NBL training run begins hours before the astronaut enters the water. Suit technicians prepare the EMU, carefully adjusting weights to achieve proper buoyancy. Safety divers—typically 10-15 per training session—conduct briefings on the day’s objectives and emergency procedures. Test directors review the timeline for the tasks to be practiced.
Once suited up, the astronaut is lowered into the pool by overhead crane. Support divers immediately check buoyancy, making fine adjustments by adding or removing small weights. Achieving perfect neutral buoyancy is critical—if the astronaut is even slightly heavy, they’ll sink toward the bottom; slightly light, and they’ll float toward the surface, making precision work impossible.
The actual training run mirrors the planned spacewalk as closely as possible. Astronauts work through procedures step-by-step: translating (moving hand-over-hand) along the mockup, positioning their feet in portable foot restraints, using specialized tools to remove and replace components, and practicing contingency procedures for things going wrong. Safety divers shadow every move, ready to intervene if needed but generally staying back to let astronauts work independently.
The Physical Demands
Working in the NBL is exhausting. The water-filled suit provides resistance to every movement. The gloves, pressurized to simulate space conditions, make gripping tools difficult—astronauts often lose fingernails from the constant pressure of gripping through stiff glove materials. After a six-hour session, astronauts’ hands are often bruised, and their forearms burn from the constant gripping force required.
Maintaining body position in three dimensions while performing delicate tasks requires constant core engagement. Astronauts must prevent their bodies from rotating or drifting while they work, all while wearing a suit that restricts movement and vision. The helmet limits peripheral vision and makes it impossible to look directly down at one’s feet—astronauts must learn to work by feel and spatial awareness.
The physical demands of NBL training serve a purpose beyond skill development—they build the endurance needed for actual spacewalks. A six-hour EVA in space is similarly exhausting. While there’s no water resistance, the pressurized suit still resists movement, and working in microgravity requires constant muscle engagement to prevent unwanted rotation and maintain body position.
Support Divers: The Unsung Heroes
Safety divers play essential roles during NBL training. Typically former military divers or civilian diving professionals with extensive experience, these divers undergo specialized training to support astronaut operations. During a training run, divers perform multiple functions simultaneously.
Buoyancy control divers make continuous fine adjustments, adding or removing small weights to maintain perfect neutral buoyancy as the astronaut moves through the water. Translation divers help astronauts move between workstations, simulating the ease of movement in actual weightlessness where a gentle push sends you gliding. Tool divers retrieve and position tools, simulating the flight crew members who would assist during actual spacewalks.
Safety divers also watch for problems: tangled safety tethers, uncomfortable suit positions that could cause injury, or signs of fatigue or distress. They’re trained to recognize when an astronaut needs a break (even if the astronaut doesn’t want to admit it) and can immediately end a training session if safety concerns arise.
From Pool to Space: Translating Training to Reality
The transition from underwater training to actual spacewalks isn’t seamless—astronauts must adjust to differences between the two environments. In space, there’s no water resistance, so movements that require force underwater happen with unexpected speed in orbit. Astronauts who push off too hard from a surface can find themselves floating away from the ISS, requiring a careful pull on their safety tether to return.
Temperature is another major difference. The NBL water stays around 82-84°F for diver comfort, but space presents extreme temperature swings—from +250°F in direct sunlight to -250°F in shadow. Astronauts can’t feel these temperature changes through their suits, but they must learn to manage them by planning work to avoid overheating or having tools freeze.
Despite these differences, astronauts consistently report that NBL training prepares them well for spacewalks. The procedures, tool manipulations, and body positions they practiced underwater translate directly to orbit. When unexpected challenges arise during EVAs, astronauts draw on their NBL experience to improvise solutions.
Beyond the ISS: Training for Future Missions
As NASA prepares for Artemis Moon missions, the NBL is adapting. New mockups simulate the Lunar Gateway space station and lunar surface operations. While the Moon’s one-sixth gravity differs significantly from the microgravity of orbit, underwater training still helps astronauts learn procedures and practice with tools.
For Mars missions, the NBL will face new challenges. Mars has about 38% of Earth’s gravity—too much for neutral buoyancy simulation. NASA is developing augmented reality systems that can overlay virtual Mars terrain onto pool training, helping astronauts prepare for working in partial gravity while still benefiting from underwater practice.
The fundamental principle remains: practice in the most realistic environment available, repeat until procedures become automatic, and prepare for everything that could go wrong. These lessons from NBL training will serve astronauts whether they’re working outside a space station, on the Moon, or eventually on Mars.
Conclusion
The Neutral Buoyancy Laboratory represents one of NASA’s most important training facilities. In this giant pool, astronauts transform from students learning procedures to practitioners ready to work in the most unforgiving environment humans have ever entered. Every successful spacewalk—whether replacing a failed component, installing new experiments, or upgrading station systems—traces its success back to hours spent underwater in Houston.
The next time you watch astronauts floating outside the ISS, consider that every movement, every tool manipulation, and every procedure was practiced extensively underwater. The casual efficiency with which they work belies the intensive preparation that made it possible. The NBL doesn’t make spacewalks safe—nothing can make working in vacuum truly safe—but it makes them survivable, manageable, and ultimately successful.
For the astronauts who train there, the NBL is both challenging and rewarding. It’s where they prove they have what it takes to work in space. It’s where theory becomes practice, and practice becomes instinct. And it’s where the human dream of working among the stars takes one crucial step toward reality.
Related Articles
• Spacewalks: The Most Dangerous Work Astronauts Perform
• The Extravehicular Mobility Unit: Inside the Spacesuit
• ISS Maintenance: Keeping the Station Operational
• Return to: Astronauts, Training, and Life in Orbit