With the success of the Apollo lunar exploration program of the 1960s and early 1970s, NASA shifted its attention to developing a twopronged human spaceflight program. One project was establishing a space station in long-term orbit, which the agency first accomplished with the launch of Skylab. It housed only three crews for a total of 171 days in 1973, but it paved the way for later stations. The other project was developing a reusable space vehicle. The Space Transportation System (STS), popularly known as the space shuttle, could act as a ferry to future space stations as well as allow astronauts to stay in orbit for weeks at a time as they conducted scientific research and launched or repaired satellites.

Compared to earlier spacecraft, the five space shuttle vehicles NASA and its contractors built from 1972 to 1991 are almost unimaginably complex. (A sixth...