Astronauts outside the shuttle are encumbered by a heavy spacesuit, but this is not really a problem in weightless conditions. Controlled movement is more difficult, however. Astronauts wear a unit called a manned maneuvering unit (MMU) on their backs. This is fuelled by nitrogen and is rechargeable in the shuttle. Several small thrusters allow the astronaut to move in all directions.
The process leading up to a spacewalk takes almost an entire day, and it’s not because of the many items that make up an astronaut’s suit. The main reason for this is because astronauts need time to go through decompression, the same procedure cave divers use when returning from the depths of the ocean to the surface of the water.
To allow maximum mobility and maximum protection from the lack of pressure in space, a spacesuit is pressurized at 29.6 kilopascals during a spacewalk, about one third of the pressure experienced by the crew inside the spacecraft. Astronauts also have to breathe in pure oxygen, because the amount of oxygen in air at such a low pressure isn’t enough.
Now, if an astronaut simply donned a space suit in 15 minutes and promptly exited an airlock, he or she would go through decompression sickness, or “the bends” — the same thing scuba divers experience if they’re exposed to a rapid drop in external pressure by ascending too quickly. The bends causes expanded nitrogen gas bubbles in the bloodstream to escape too quickly, and joint pain, dizziness, cramps, paralysis and even death can follow.
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