Neutral Buoyancy Laboratory
The Neutral Buoyancy Laboratory (NBL) is an astronaut training facility maintained by and located at the Sonny Carter Training Facility near NASA's Johnson Space Center in Houston, Texas. The NBL consists of a large indoor pool of water, in which astronauts may perform simulated EVA tasks in preparation for upcoming missions. The NBL contains full-sized mock-ups of the International Space Station (ISS) modules and payloads, visiting vehicles such as the Japan Aerospace Exploration Agency (JAXA) HTV, the European Space Agency ATV, the SpaceX Dragon, and the Orbital Sciences Corporation Cygnus. Previously there was also a mockup of the Space Shuttle payload bay, but since Space Shuttle retirement it has been removed.
The principle of neutral buoyancy is used to simulate the weightless environment of space. First the suited astronauts or equipment is lowered into the pool using an overhead crane. Once this is done the suited astronauts weighted in the water by support divers so that they experience minimal buoyant force and minimal rotational moment about their center of mass. The suits worn in the NBL are down-rated from fully flight-rated EMU suits like those in use on the space shuttle and International Space Station.
The NBL tank itself is 202 feet (62 m) in length, 102 feet (31 m) wide, and 40 feet 6 inches (12.34 m) deep, and contains 6.2 million gallons (23.5 million litres) of water. Divers breathe nitrox while working in the tank.
One downside of using neutral buoyancy to simulate microgravity is the significant amount of drag presented by water. This makes it difficult to set an object in motion, and difficult to keep it in motion. It also makes it easier to keep the object stationary. This effect is the opposite of what is experienced in space, where it is easy to set an object in motion, but very difficult to keep it still. Generally, drag effects are minimized by doing tasks slowly in the water. Another downside of neutral buoyancy simulation is that astronauts are not weightless within their suits, meaning that as an astronaut tilts the suit they are forced to lean against whatever inside surface is facing down. This can be uncomfortable in certain orientations such as heads-down. Thus, precise suit sizing is critical.
Alternative microgravity simulators 
The other primary method used by NASA to simulate microgravity is the so-called "Vomit Comet", an aircraft which performs a number of parabolic climbs and descents to give its occupants the sensation of zero gravity. The vomit comet reduces the problem of drag in weightless simulation. The main shortcoming of this method is its time limitations - periods of weightlessness are limited to around 25 seconds, interspersed with periods of acceleration of around 2 g as the aircraft pulls out of its dive and readies for the next run. This is obviously not suitable for practicing EVAs, which usually last several hours.
Non-NASA, non-US-government neutral buoyancy facilities 
The Neutral Buoyancy Research Facility at the University of Maryland's Space Systems Laboratory performs research into EVA techniques and robotic interaction using neutral buoyancy as a basis for weightless simulation. Equipment is lowered into the pool using an overhead crane.
See also 
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- "NBL Characteristics". About the NBL. NASA. June 23, 2005.
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- Rafiq A, Hummel R, Lavrentyev V, Derry W, Williams D, Merrell RC (August 2006). "Microgravity effects on fine motor skills: tying surgical knots during parabolic flight". Aviat Space Environ Med 77 (8): 852–6. PMID 16909881. Retrieved 2008-08-27.
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- University of Maryland. "Space Systems Lab: Neutral Buoyancy Research Facility". Retrieved 2008-08-27.
- Science discovery channel. "How do they do it". Retrieved July 17, 2009.