During the first nine space station assembly missions, US astronauts can't take space walks with the space shuttle present. The Russian Service Module provides a capability for station-based Russian space walks using only Russian spacesuits, but the U.S. capability will not be available until the Joint Airlock Module is attached to the station.
The Joint Airlock Module, which supports both Russian and U.S. spacesuit designs, consists of two sections. There is a crew lock astronauts use to exit the station and begin a space walk and an equipment lock for storing gear. The crew will also use the equipment lock overnight "campouts." The astronauts will lower the pressure in the Joint Airlock Module to 10.2 pounds per square inch (psi), while the rest of the station remains at the normal sea level atmospheric pressure of 14.7 psi. The night spent at 10.2 psi in the airlock purges nitrogen from the space walkers' bodies and prevents decompression sickness, commonly called "the bends," when they go to the 4.3 psi pure oxygen atmosphere of a spacesuit. Station crew members could perform a space walk directly from the 14.7 psi cabin atmosphere, but they would have to breathe pure oxygen for several hours first. The Airlock "campout" shortens the pure oxygen prebreathe time to only minutes for the crew. The protocol is similar to a procedure commonly used in advance of space shuttle space walks in which astronauts lower the shuttle's cabin pressure to 10.2 psi at least a day ahead of the EVA.
After the Joint Airlock Module is operational, the philosophy of space walk training will shift due to the increasing complexity of the station and the ability of the station crew to perform space walks. Rather than attempting to train station crew members for every EVA task they may be called upon to perform during a mission, training will increasingly aim toward providing crew members with a general suite of EVA skills. The station's growing size and complexity will make it virtually impossible for astronauts to train for every possible contingency and maintenance EVA, as is the case in training for Shuttle missions.