293 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 Extravehicular Activity Testing and Training Figure 13. The NBL near Johnson Space Center. Above the water level is the control room, a mini Mission Control, visible on the far right behind the white wall. The yellow cranes are used to hoist crew or other test subjects into the water. The crew must use special EMUs that are made to be used in the water. The mock-ups underwater include the Laboratory Destiny with the central piece of the truss structure on top (farthest away), truss and solar array structures (center and right), and the Space Shuttle’s cargo bay (foreground). Figure 14. An astronaut (i.e., the test subject) translates along the forward face of the mocked-up ISS truss, with a scuba diver assisting to keep the air and cooling umbilical from becoming entangled or pulling on the EMU. Testing and crew training for an EVA takes place in many facilities due to the complexities associated with adequately mimicking microgravity, working in a spacesuit, and the large scale of the ISS. One of the main facilities used for EVA development and crew training is the Neutral Buoyancy Laboratory (NBL)—a large pool that measures approximately 61 m (200 ft) long by 30 m (100 ft) wide by 12 m (40 ft) deep—near Johnson Space Center in Houston, Texas (Figure 13). Training versions of EMUs are balanced by scuba divers with weights so the test subject does not sink or float and can work as he or she would in zero gravity. The underwater suits each use an umbilical hose that goes to the surface to supply breathing gas and cooling. The divers manage this umbilical, thus the crew usually does not know it is there. Water does not provide a perfect model for space— the ISS mock-up and tools corrode over time, the water drag makes large objects (including the crew members) harder to start moving and easier to stop moving than in space, and equipment is inclined to float or fall in a manner that it would not in space. Also, the ISS is so big that the entire structure does not fit in the pool, so it is broken into pieces. When the crew members translate along the ISS structure in the pool, the divers must assist by physically moving the crew members between parts of the ISS when there is a gap between structures (Figure 14). The NBL replaced the Weightless Environment Training Facility at