283 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 astronaut “falls off” of the ISS since the space station is not able to chase after a lost crew member as the Space Shuttle theoretically could have done. The focus of this chapter is on US spacewalks using the US spacesuit, which is called the Extravehicular Mobility Unit (EMU). However, Russian EVAs (using the Orlan spacesuit out of a Russian Segment airlock) have also contributed greatly to the construction and maintenance of the ISS. US EVA experts spent a number of years during early ISS construction temporarily living in Moscow to work with Russian EVA experts for the purpose of negotiating crew training, spacewalk techniques, and hardware use. Spacewalks have been conducted by Americans in Orlans and Russians in EMUs (plus other international partners in both suits), and training has been conducted in both countries in both spacesuits. In fact, early in the program, it was envisioned that the US Segment airlock would be used for both Orlan and EMU EVAs. The airlock was built accordingly to accommodate both suits. The airlock Quest on the US Segment is called the Joint Airlock for that reason. However, having Orlan operations on the US Segment has not been needed as much as anticipated, and Russian Segment tasks are closer to the Russian airlock. Therefore, some of the equipment that the Joint Airlock would require for an Orlan EVA was not launched for installation in the airlock (notably, the Orlan umbilicals needed for oxygen, cooling, power, and communication while the crew member was still in the airlock). All Orlan EVAs have been executed out of Russian Segment airlocks, to date. The US Spacesuit The iconic white US spacesuit has to provide the functions of a spacecraft while being wearable. Its design is mostly unchanged from the Space Shuttle Program, although some features and components evolved during ISS operations. For example, the Space Shuttle EMUs were designed to be used for the short duration of a shuttle mission—i.e., no more than a handful of EVAs—and returned to Earth for servicing before use on a subsequent flight. It was desirable to leave the EMUs on the station for longer periods of time since launching bulky spacesuits over and over to the ISS is expensive and would mean other things couldn’t be launched in their place. The spacesuit contractor extended the life of EMUs by replacing some parts and testing others for longer life to enable them to remain on orbit for several years without periodic ground checks. Another upgrade was to toughen some of the material on the gloves. Astronauts use their hands to maneuver around the ISS, thus the glove material had to be strengthened to withstand extended wear and provide more protection against the sharp edges that develop on the exterior of the station over time. Although almost all of the external hardware is manufactured to have smooth edges, Micrometeoroid and Orbital Debris (MMOD) strikes on the ISS surface over the years have produced various cuts and protrusions in the metal handholds and on other surfaces. The material upgrade was made after the crew discovered a number of cuts in the outer layer of the gloves following EVAs. The EMU (Figure 3) provides the fundamental needs for a crew member on a spacewalk lasting approximately 6.5 hours. It provides pressure and an oxygenated atmosphere, environmental protection from extreme temperatures/radiation/ some space debris, mobility, and communications. Sometimes, the EMU can provide these fundamental needs for more than 6.5 hours, depending on the thermal environment, specific crew member metabolic rate (which determines how much oxygen he or she breathes and how much carbon dioxide is produced), and the difficulty or workload for a given EVA. In addition to the expected 6.5 hours, the suit allows the team to manage 30 minutes of additional capability (e.g., oxygen and other consumables, such as battery power, that are used during an EVA) as pure margin to ensure the crew member gets back inside the station before critical supplies run out. The EMU breathing environment during an EVA is pure oxygen at 217 millimeters of mercury (mm Hg) (4.3 pounds per square inch [psi]), which is equivalent to approximately 9 km (30,000 ft) altitude simply in terms of pressure. This environment was originally chosen based on historical knowledge of pressure suit systems, and it accommodated several competing requirements for Space Shuttle EVAs. A spacesuit needs a strong pressure bladder to retain the atmosphere, as well as restraining material to conform the suit to a human and hand shape. Lower pressure is necessary to allow for the mobility required to grasp tools and move around the structure while working inside the bladder and restraints. Even at