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What is "Neutral Buoyancy"?

IMAGE: Flotation devices

Small flotation devices strapped to an astronaut's ankles help to simulate microgravity as he practices a spacewalk in the pool.

"Neutral buoyancy" is a term used to describe something that has an equal tendency to float as it does to sink. This effect is accomplished with a combination of weights and flotation devices.

Articles that are configured to be neutrally buoyant seem to hover under water, and large, neutrally buoyant items can be easily manipulated much like in orbit. However, there are two important differences.

First, a suited astronaut in the Neutral Buoyancy Lab, or NBL, is not truly weightless. While it is true the suit/astronaut combination is neutrally buoyant, the astronauts feel their weight while in the suit. They are lying or standing in the suit, depending on its orientation, which is one reason why suit fit is so critical.

Second, water drag acts to hinder motion. This makes some things easier to do in the NBL than in space and some things more difficult. Both effects are unlike the conditions of space and must be recognized during spacewalk training. However, even with these limitations, neutral buoyancy is currently the best available method for long-duration spacewalk training.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 09/30/2002
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