On May 5, 1961, U.S. astronaut Alan Shepard spent 15 minutes skimming the edge of space, making him the first American and second human to ever leave the planet.

Now, 50 years later, all U.S. space shuttles have officially retired, bookending a half-century of manned space flight that forever changed the course of human civilization. When Atlantis returned to Earth on July 21 after its final mission to the International Space Station, it marked not only the end of NASA's 30-year shuttle program, but also its five-decade experiment of putting people in space.

The space agency is now beginning a period of upheaval and uncertainty, shifting some of its more routine space-flight tasks to commercial firms like SpaceX and Virgin Galactic, while searching for a new identity as it refocuses on new goals in deeper space.

While NASA sets its sights on Mars, comets and asteroids, however, it's worth taking a look back at how we got here — how, within one generation, humanity went from merely dreaming about space to building a livable, even comfortable outpost there. To learn more about the growth of this world-changing ambition, check out the interactive graphic at bottom. And for more information about NASA, the space shuttle program and other cosmic issues, see the video below:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Interactive: 50 years of Americans in space
As NASA's space shuttle program comes to an end, it marks a turning point in a half-century of manned space flight. MNN takes a look back at five decades of U.S