299 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 bloodstream to critical organs, resulting in pain, severe medical issues, and even death. EMU Life Support System Backpack SAFER tower with upper thrusters at height of crew’s shoulders (second tower on crew’s right side) right side) SAFER main body sits under the EMU life support, housing lower thrusters and nitrogen Figure 22. SAFER installed during a test on Space Shuttle flight STS-64. Multiple prebreathe protocols have been available and used on the Space Shuttle and the ISS. The various protocols can involve oxygen masks, exercise in the suit or on a bicycle machine, reduction in cabin pressure to 528 mm Hg (10.2 psi) (~equivalent to 3 km [10,000 ft] altitude) for a period of time, and/or prebreathe in the spacesuit. Prebreathe methods have evolved to incorporate reductions in crew day length and reduced complexity. For example, at one point, prebreathe (Figure 23) involved having the EVA crew sleep in the Joint Airlock overnight at a reduced cabin pressure however, this had its pitfalls. In addition to sequestering the extravehicular crew members from their crewmates and the toilet, certain failures such as a fire alarm on the ISS will cause a repress of the airlock and will interrupt the prebreathe process. This can be frustrating when the alarm is false, as has happened in the past, since any interruptions in prebreathe protocol require strict penalties to “buy back” the time. Depending on the situation, the crew might have to breathe pure oxygen for twice the number of minutes than was the interruption. Figure 23. Astronaut Chris Cassidy is shown prior to an EVA during one portion of his prebreathe of pure oxygen to prevent DCS. In addition to an oxygen mask, he is wearing his LCVG, with small water- filled tubes throughout and larger ventilation ducts, as seen on his left arm and on his right waist. The Crewlock that goes to vacuum (located behind Cassidy) is filled with tools and storage bags. The suit umbilicals are attached to an umbilical interface panel in the Crewlock on one end and the EMUs on the other end. The EMUs are in the Equipment Lock in the foreground. One suit umbilical is routed under Cassidy’s arm and is connected to a suit, seen in the lower right corner of the photo. If a crew member exhibits DCS symptoms during an EVA, he or she will be brought inside as quickly as possible (with assistance from the other crew member, as required). The airlock will be repressurized, which immediately aids in recovery. The suit