Video Transcript:

Why Do Astronauts Practice Underwater?

Alexandra: Have you ever tried to hold an inflated ball underwater? The ball stays afloat because of buoyancy. The ball floats because air is less dense than water, and a piece of steal sinks because steal is more dense than water. But a whole ship made of steel doesn't sink because the ship and everything inside it, including air, has a lower average density than water.

Astronauts use the same principle to simulate weightlessness while they train in a giant swimming pool known as NASA's Neutral Buoyancy Laboratory. An astronaut wearing a 73 kilogram spacesuit doesn't sink to the bottom because the astronaut, the suit and the air inside all make a system that is less dense than water, so it floats.

SCUBA divers add weights to make the astronaut “neutrally buoyant,” so the astronaut doesn't float or sink while they practice their spacewalk.

Even though there's some resistance moving their arms and legs in the water, it's still the closest thing on Earth…to walking in space.

There's more on the underwater world of astronauts, at nasa.gov.

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NASA Brain Bite BB 1101B