In the early twenty-first century, more than twenty years after the successful maiden voyage of space shuttle Columbia, the National Aeronautics and Space Administration's (NASA) Space Transportation System—commonly known as the space shuttle—remains the only U.S. transit system capable of supporting human spaceflight. The shuttle is also the world's only largely reusable launch vehicle, comprised of an airplane-shaped orbiter, which returns to Earth with its human crew for refurbishment and re-flight; two solid rocket boosters, which are recovered for reuse after they separate from the rest of the shuttle system during ascent to orbit; and an irrecoverable fuel tank. (In contrast, the stages of so-called expendable launch vehicles—which launch most of the world's satellites as well as passenger-carrying Russian Soyuz capsules—are jettisoned to disintegrate in Earth's atmosphere or are left as debris in space.)

Capable of sustaining...