303 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 it back into the cabin. Several times, the spacewalking crew has been exposed to ammonia. The potential to bring ammonia back inside can be reduced by requiring the crew “bake out” before coming through the airlock (i.e., spend additional time outside to allow sublimation of the ammonia into vapor form, off the suit). If there is doubt that crew members might still have ammonia crystals on their suits, NASA may elect to have the crew use a piece of sampling equipment called a Draeger tube to test for the presence of ammonia. Partially repressed airlock air would mix with ammonia on the crew member’s suits and tools, and visibly turn reactant chemicals in the glass vial tube from yellow to blue. Equipment that is “dropped” overboard due to missing a tether connection or other problem is tracked by ground radar, if the item is big enough to be picked up on radar. However, there is potential for these objects to re-contact the ISS, contact another vehicle approaching or leaving the ISS, or in some cases survive reentry into Earth’s atmosphere and potentially harm people on the ground. Also, the now-missing equipment may be critical to the repair or task at hand, and the team will have to determine whether it can be accomplished on that EVA. Lost equipment in the past has included tools, a bag full of tools, and a camera. Occasionally, the ISS Program will approve an object to be dropped overboard intentionally as a jettison if it can be thrown in a particular direction and proven to result in extremely minimal risk to the ISS and people on the ground. The largest object ever jettisoned from the ISS was the Early Ammonia Servicer, a 544 kg (1,200 lb) tank of ammonia in 2007. The US Segment assembly of the ISS often involved robotic installation or partial installation of a major element when a Space Shuttle first arrived, followed by a series of three to four EVAs to structurally secure the element, hook up power and data connections, and perform related tasks. These EVAs were sometimes nail-biters, because a small issue had the ability to trip up the team and amount to an element freezing or the inability to fully safe the systems (Chapter 4). The mission time was limited and the Space Shuttle crew was specifically trained for some of the tasks, so changes to the EVAs during the docked Space Shuttle timeframe were often added into that short period with incredibly fast EVA development time. Over the years, many small issues (e.g., stuck bolt that would not release) as well as major issues (e.g., the entire tray of electrical and fluid lines that would not initially deploy while the equipment was waiting for power) were overcome by the crew and ground team to complete the mission at hand. Post-assembly, the intensity remains for many tasks, especially those that involve repair of a critical ISS systems unit. One example of critical EVAs to repair the ISS is the set of two EVAs that followed the external pump failure that occurred in 2013, which required the EVA team and crew to prepare and execute EVAs within a period of a couple of weeks. Further details of this situation are provided in Chapter 20. International Space Station Extravehicular Activities— A Benefit to Humanity This chapter was written as a general overview, but the real ISS hardware build-up for Space Station assembly transpired with extremely great detail at all steps along the way. Every part of the EMU is carefully maintained and rotated. Every aspect of every EVA is carefully planned (to the extent possible on Earth) and executed. The team of people involved is extensive, from the ISS hardware engineers to the suit engineers to the divers in the NBL—and the dedication to safety and great detail from everyone is awe-inspiring. Two chapters in this book (Chapters 18 and 20) highlight the critical role an EVA can have in keeping the ISS functioning. A peripheral benefit to the experience is also gained by building the ISS by hand. The intensity, quantity, and complexity of ISS EVAs could be considered a drop in the bucket compared to a program such as an exploration mission to Mars. The ISS provides many benefits to society. Through the ISS, NASA is learning ways to improve EVA efficiency, reduce overhead in preparations, and improve spacecraft design so that fewer failures occur. The training and experience gained in doing these low- Earth orbit EVAs will be invaluable for exploration of other worlds.