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How is the external airlock used for extra vehicular activities? The airlock that we use for going on an extra vehicular activity, or an EVA, or sometimes called a spacewalk used to be located inside the crew module of the orbiter. But when we started flying to the Russian Space Station, MIR, and then going to the International Space Station,we took it out of the crew module and put it in the payload bay with a small tunnel connecting to the crew module. We also placed on top of it an orbital docking system made for us by the Russians. Now when you want to go on an EVA, the first thing that the astronauts have to do is a pre-breathe. They’ve got to get all of the Nitrogen out of their blood stream just like if you were a scuba diver. So they pre-breathe pure oxygen for several hours; they even lower the pressure in the crew module to help get that Nitrogen out. But once they are ready, they go inside the airlock where they have two Extra Vehicle Mobility Units, or EMU’s we call them, a third astronaut will help them don this very bulky space suit. Once we put the suit, we do a pressure check, the third astronaut leaves, we close the hatch and then we depressurize inside the airlock. The astronaut can open the hatch and out they go to do their EVA. And that’s how we use the airlock for Extra Vehicular Activities.