crew was limited to two. However, when the flights expanded to four and later to five and then eight, it was decided to cross-train on the same flight for many of the functions because there were not enough astronauts and training resources to fully provide for backup crews. Two pilots with dual shuttle controls provided redundancy in space shuttle landing, which turned out not to have been needed. Mission specialists were generally cross-trained for on-orbit operations, with two notable exceptions: the flight engineer position (MS2) and for extravehicular activity (EVA), which required suits of a particular size. Each space shuttle flight required at least two astronauts qualified for EVA. EVA is also a requirement on the ISS. During this period, if an astronaut had to be replaced, it was assumed that an astronaut who had flown would be drawn from those awaiting their next flight assignment (Figure 1.1).

In addition, the composition of the Astronaut Corps has evolved substantially as well, and the current corps is a technically diverse group of people who have widely varied backgrounds and experience. The astronaut program began with only military test pilots, progressed to include scientist-astronauts during Apollo and Skylab, and now is composed of test pilots and mission specialists (engineers, scientists, and physicians) and educators and international partner astronauts sent by their home agencies to train alongside their U.S. colleagues.

The major changes in the professional Astronaut Corps have corresponded with the introduction of new spacecraft designs, with the addition of research to the mission objectives, and with the adoption of new policy goals for the nation’s human spaceflight programs. The Space Shuttle program, by design, was opened to women and a more technically diverse set of engineers and scientists (mission specialist astronauts and payload specialists).

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FIGURE 1.1 The first U.S. spacewalk was performed by Ed White during the Gemini 4 mission on June 3, 1965. The addition of new tasks, such as spacewalks, increased astronaut training requirements. SOURCE: Courtesy of NASA.



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