A spacewalk is one of the most demanding tasks in human spaceflight. Astronauts work for hours in a pressurised suit, outside the spacecraft, in weightlessness. To prepare, they train underwater, where neutral buoyancy comes closest to the conditions in orbit. The centre of this training at NASA is the Neutral Buoyancy Laboratory in Houston.
The Neutral Buoyancy Laboratory
The Neutral Buoyancy Laboratory (NBL) is part of the Sonny Carter Training Facility at NASA's Johnson Space Center. According to NASA, the pool is 202 feet (61.5 metres) long, 102 feet (31.1 metres) wide and 40 feet (12.2 metres) deep and holds 6.2 million gallons of chlorinated fresh water kept at 84 to 86 degrees Fahrenheit. It is used for mission planning, procedure development, hardware verification and astronaut training. Professional divers using SCUBA and surface supplied dive systems provide safety and operations support.
How neutral buoyancy works
Astronauts in training suits are weighted so that they neither sink nor float. This lets them practise moving along structures, handling tools and working with large components in conditions similar to weightlessness. NASA describes extensive experience in manipulating buoyancy to simulate full or partial gravity, which also matters for training future Moon missions.
A short history of underwater training
- 1966: Buzz Aldrin became the first astronaut to use neutral buoyancy to train for spacewalks, in a school pool with mockups of the Gemini and Agena spacecraft. Less than a month later, he completed a then-record three spacewalks on Gemini XII.
- Skylab: astronauts trained in the larger Neutral Buoyancy Simulator at NASA's Marshall Space Flight Center.
- 1980 to 1997: the Weightless Environment Training Facility in Houston was used for nearly all Space Shuttle spacewalk training.
- 1996 and 1997: the first suited dives in the NBL took place in October 1996, and the facility was formally dedicated on 19 May 1997.
More than the pool
Spacewalk preparation also includes suit and airlock procedures. The cover image shows Canadian astronaut Joshua Kutryk of Crew-13 practising suiting up in an International Space Station airlock simulator at Johnson Space Center's Space Vehicle Mockup Facility in May 2026. More on astronaut preparation is in our guide to how astronauts train.
Frequently asked questions
Where do astronauts train for spacewalks?
At NASA, mainly in the Neutral Buoyancy Laboratory at Johnson Space Center in Houston, a pool holding 6.2 million gallons of water.
How deep is NASA’s astronaut training pool?
According to NASA, the Neutral Buoyancy Laboratory is 40 feet, or 12.2 metres, deep.
Who first trained for a spacewalk underwater?
Buzz Aldrin, in 1966, before his Gemini XII mission.
Related reading
Sources
- NASA: Neutral Buoyancy Laboratory
- NASA History: Building on a Mission, Neutral Buoyancy Facilities for Spacewalk Training
- NASA Image Library: jsc2026e037769
Cover image: CSA astronaut Joshua Kutryk practises suiting up in an ISS airlock simulator at Johnson Space Center, 5 May 2026. Credit: NASA/Josh Valcarcel (jsc2026e037769), public domain.