CHAPTER 17 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION 302 keep from disengaging and floating away from the robotic arm. The crew members would remain attached to the ISS via their long retractable tethers, but this would be quite a debacle if a crew member was carrying a large piece of hardware. Risks to the ISS occur during EVAs due to the very nature of the human element and the potential for additional failures. A suit problem could force an EVA to be terminated (ended expeditiously) or aborted (ended extremely quickly in an emergency fashion to save a crew member’s life). For example, the control team terminated an EVA during STS-118/ISS-13A.1 (2007) due to unexpected glove damage, and during US EVA 23 (2013) when water entered a crew member’s helmet (see sidebar: US Extravehicular Activity 23 Water- in-Helmet Incident). In some cases, ending an EVA early could mean that bags or tethers block rotation of critical appendages such as the solar arrays or could block robotic movement. Also, if a large piece of the ISS is not tied down adequately with tethers, then it is possible that vehicles such as Soyuz could not dock or undock safely without the risk of having equipment fall off and impact other structures or vehicles. The EVA console in Mission Control Center (MCC) provides a status to the flight director and flight control team on the amount of consumables left in each EMU. The consumables include the oxygen quantity in the primary tanks, the capacity of the CCC in terms of ability to continue to remove carbon dioxide, water available for sublimator use, and battery power. The quantity of available oxygen is directly measured and sent to MCC in telemetry. The CCC does not put out telemetry of its capacity. This is derived based on how much oxygen a crew member is using, which is an indicator of his or her metabolic rate. From this metabolic rate over the course of the EVA, the team can determine the remaining CCC capacity based on the predicted capacity and subtracting out the already-used quantity. Water usage is not known until the reserve water comes on (automatically turned on due to reduced line pressure when the primary water is used up). In that case, the crew and ground will receive an alert that 30 minutes remain, assuming a hole is not causing leakage, which would reduce the available time. Battery power is predicted and voltage can be measured for unexpected reduction in the remaining time available. The flight control team works to ensure the critical tasks get done while keeping an eye on consumables and crew health. Sometimes, predicting what the consumables will be is a challenge in this regard. For example, the limiting consumable (the one with the least amount of time remaining) is often the CCC. However, a crew member’s metabolic rate varies over the course of an EVA, with lower metabolic rates when on the Space Station Robotics Manipulator System. But high-effort tasks increase metabolic rate. Thus, determining whether a task is achievable in that EVA requires some predictive skills before an astronaut starts the task. Training runs in the NBL help establish trends for specific crew members. Oxygen can be recharged (refilled) by having the crew go into the airlock and connect to the umbilical, although this could significantly alter the choreography of the EVA. EVAs involve the human element both in space and on the ground. Crew member height and arm length can make a big difference in whether that crew member can perform a particular task. Mechanical aptitude is critical as well. The ISS needs someone who knows how to turn a wrench as much as they know science. Some complex tasks require very good back-and-forth communication between the crew and the ground, so language and patience are also factors. Assembly mission EVAs used the on-board crew (called “IV” for intravehicular) to read the instructions to EVA crew members and keep them on task for standard increment operations EVAs, a “Ground IV” relays the details of each step from MCC. EVAs are physically demanding therefore, crew fatigue is a major consideration during a spacewalk. Sometimes an EVA will go longer than planned because a task at the end of an EVA takes longer than anticipated due to crew member fatigue. Ground discussion among themselves and what information is exchanged with the crew on board is often key in decision making and the success of an EVA. Exposure to toxic chemicals during a spacewalk is another hazard that is carefully managed. The external coolant on the ISS is ammonia that, even in very small amounts, could be lethal if it gets stuck on the EMU or tools and the crew member brings