STS-103
Astronaut Neutral Buoyancy Testing

To prepare for a Hubble servicing mission, astronauts spend many hours training underwater. This allows them to experience how their suits, science instruments, tools, crew aids and other equipment respond to movement in a space-like environment. In the water, astronauts wear small weights on their space suits to make themselves "neutrally buoyant." This means they neither sink nor rise, but hover suspended, as they would in space. By practicing their servicing tasks underwater, the spacewalkers learn to work swiftly and efficiently in orbit.