295 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 Figure 16. Astronaut Dan Burbank flies a mocked-up SAFER unit in the Virtual Reality laboratory. He is seeing the space station in 3-D and using the hand controller to “fly” back to the ISS. This provides a simulation of what it would be like if his tether broke and he detached from the structure. All told, the preflight EVA development process is fairly lengthy and complex. In an attempt to make designs EVA-friendly, with handholds and interfaces for standard EVA tools, NASA levied requirements on the ISS hardware designs. The operations team often works with the design team during development. Analysis and preliminary design would often lead to building mock-ups for preliminary testing underwater. EVA teams might test choreography for an EVA several times in the NBL. The teams also go to different facilities around the country to test the fit of tools, put together pieces of the real ISS, and try out putting blankets on structure—all things that cannot be done realistically underwater. Astronauts typically get into a spacesuit and go through the procedures for depressing the airlock to get a feel for the stiffness and sounds of a flight-like spacesuit. The crew slated for a spacewalk will practice flying SAFER in the Virtual Reality laboratory, as well as practice calling for robotic movement from the crew inside (e.g., “Move me down to the structure”). For assembly, crews would get into the real Space Shuttle cargo bay where ISS elements were located, as well as look at Space Shuttle interfaces in case of Space Shuttle off-nominal situations that might need an EVA. Astronauts may participate in other specialty classes and tests associated with tools or ISS hardware to further prepare them for a plethora of situations. After all is said and done, the EVA development effort involves many operations experts, hardware and EVA tool designers, analysts, experienced astronauts, safety experts, and facility experts. Extravehicular Activity Tools Tethers (cords) are critical for keeping hardware from floating away and act as lifelines back to the ISS structure (Figure 17). Some tethers are retractable and can be temporarily locked out (similar to a measuring tape), while others are a fixed length. Tethers are a constant source of discussion and can be key to the choreography, so the team carefully considers where a tether is best anchored on the crew member or structure. Work sites are often farther from the airlock than the length of a single tether, thereby requiring multiple tethers to be strung together or used in combination, which increases the complexity of getting somewhere and ensuring the astronauts always “make” (i.e., close the hook for) a connection before they “break” (i.e., open the hook from) the previous connection. Occasionally, a crew member can end up in a “snarl” or get snagged by a cord. In these cases, the astronaut must carefully untangle himself/ herself or the equipment, although the EVA choreography is designed to prevent such tangling. Crew members often use foot restraints when they need to work on something with two hands. In microgravity, actions such as pushing a piece of equipment or a tool would cause the astronaut to float in the