281 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 The Foundation for International Space Station Spacewalks Spacewalks—or extravehicular activities (EVAs)—performed on the International Space Station (ISS) evolved from a rich history of spacewalk experience starting in 1965 with the first Soviet and American spacewalks and continuing through subsequent human space programs. Spacewalks continued to evolve when the ISS came along since building the station required an unprecedented number of EVAs. The ISS is almost four times as large as the Russian space station Mir and about five times as large as the US Skylab, with a design that demanded a lot of manual bolting together of components and hand-mated connections. As the timeline for construction of the ISS approached, EVA teams looked at the daunting task of assembly as the “Wall of EVA” (Figure 1) since the amount of EVA time would rise rapidly compared to prior years. As of Expedition 51 in 2017, astronauts had completed more than 1243 EVA hours in 191 EVAs for the ISS, including 28 Space Shuttle-based EVAs. Astronauts and cosmonauts conducted roughly 80% of ISS EVAs in US spacesuits, and the rest were conducted in Russian spacesuits out of a Russian airlock. For comparison of the US programs, the Gemini Program conducted nine EVAs (the first of which was only 20 minutes long, compared to today’s standard planning of 6+ hours per EVA), the Apollo Program conducted five spacewalks and 21 lunar “moonwalks,” and Skylab conducted 10 EVAs. Prior to ISS assembly, astronauts conducted 41 EVAs during Space Shuttle flights over the course of 15 years. Looking back now at the Wall of EVA, NASA’s expectations were right on the money. These spacewalk- heavy years were extremely intense and challenging for the Space Shuttle and ISS teams. But these years were also incredibly rewarding, with the creation and growth of the space station occurring one EVA at a time. 140 120 100 80 60 40 20 0 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 Total PET (hours) Gemini Apollo Skylab Shuttle U.S. EVA Hours Per Year Shuttle First ISS-based EVAs for ISS First Shuttle EVAs for ISS Shuttle and ISS Post Challenger Accident Post Columbia Accident Year ISS Last Shuttle Flight and ISS Assembly Complete for US Segment Figure 1. US EVAs over the course of NASA’s history through mid 2017. The Wall of EVA hours looked daunting to the EVA teams as the ISS Program approached. The Space Shuttle Program provided the major basis for US EVA hardware and techniques for the ISS since the shuttle was bringing up the United States On-orbit Segment elements for installation and assembly. Furthermore, the US Segment was