CHAPTER 17 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION 298 of the suit by putting his or her arms through the suit arms mounted on a Hard Upper Torso, as if putting on a shirt over the head. The pants mate up to that Hard Upper Torso and seal in a ring around the crew member’s waist. Then, the communications cap, helmet, and gloves are donned as the suit gets powered up (Figure 20). After suit up, a leak check is performed to ensure the suit does not have any unexpected gas leakage—something as small as a human hair in between two seals will cause a noticeable leak and result in the crew having to double- check the connections. A leak check is accomplished by monitoring the pressure in the suit for 1 minute and verifying the pressure doesn’t change unexpectedly. Figure 20. Astronauts Steve Smith and Rex Walheim are suited up in the Equipment Lock during STS-110/ISS-8A. Walheim, on the right, already has a Mini-Workstation of tools installed on his chest. Jerry Ross, seen in the Crewlock in the background, helped as an intravehicular crew member for this EVA. One of the last things installed on the EMUs before an EVA is the SAFER, which the crew can use to fly back to the structure if tethering fails (Figures 2, 21, and 22). Figure 21. The SAFER on the right is attached to the EMU before the crew gets into the Crewlock for airlock depress. A hand controller, similar to a joystick, stays tucked into a storage area. If a crew member needs to rescue himself or herself by flying back to the ISS structure, the hand controller can be removed from storage quickly. Prior to each EVA, an oxygen prebreathe is conducted to prevent the crew from getting the bends after depressurization of the airlock (i.e., when the absolute suit pressure drops to 222 mm Hg [4.3 psi]). This is the same DCS that ascending scuba divers as well as aviators at high altitudes must also prevent. Breathing 100% oxygen forces nitrogen to migrate out of tissues and is exhaled. If done for long enough, the chances are greatly reduced that these gases will create harmful bubbles that can lodge in joints or travel in the