The airlock is a small room between the <a href="Glossary">pressurized</a> crew compartment and the airless vacuum of the payload bay. When a crew member needs to go into the payload bay or perform an <a href="text">extravehicular activity, or EVA</a>, the astronaut enters the airlock and puts on one of the two <a href="text">extravehicular mobility units, or EMUs</a>. Once the astronaut has put on the EMU, another crew member will slowly <a href="Glossary">depressurize</a> the airlock until it is the same airless environment as the payload bay. The astronaut in the airlock can then open the door and enter the payload bay.
