On July 21, the Space Shuttle Atlantis landed for the last time, ending three decades of missions in space. With the conclusion of the Shuttle program, the quest to restore the United States' ability to send astronauts beyond the Earth became far more urgent. Though NASA has been working on a new vehicle since 2004, it also began supporting a number of home-grown commercial space operations. As part of the Commercial Crew Development
initiative, four companies have started work on spacecraft that will eventually carry astronauts to the International Space Station -- and perhaps beyond.