More than 50 years after the Mercury astronauts made their first briefforays into the new ocean of space, Earth orbit has become a busy arenaof human activity. In that time, more than 300 people have traveled intoorbit on U.S. spacecraft. The first astronauts went along stuffed intocapsules barely large enough for their bodies, eating squeeze-tube foodand peering out at Earth through tiny portholes. Their flights lastedonly a matter of hours. Today, we routinely launch seven people at atime to spend a week living, working, and exploring aboard the SpaceShuttle. In addition to regular launches, crew members from variousnations keep a permanent human presence aboard the International SpaceStation (ISS).The history of spaceflight has seen not only anincrease in the numbers of people traveling into orbit, but also markedimprovements in their vehicles. Each successive spacecraft, from Mercurythrough Apollo and the Space Shuttle, has been larger, morecomfortable, and more capable. Scientists working inside the Shuttle’sSpacelab have many of the comforts of a laboratory on Earth, none ofwhich were available when human spaceflight first began.Someprojects, like Apollo, produced stunning firsts or explored newterritory. Others — notably, Skylab and the Space Shuttle — advanced ourcapabilities by extending the range and sophistication of humanoperations in space. Both kinds of activity are vital to establishing apermanent human presence off Earth.Almost 50 years after the dawn ofthe age of spaceflight, we are learning not just to travel into space,but also to live and stay there. That challenge ensures that the decadesto come will be just as exciting as the past decades have been.