Competing spacecraft designs must pass safety and reliability tests, as shown in this picture of the Dream Chaser concept undergoing structural testing.
The goal of supporting private manned spacecraft is to bridge the "spaceflight gap" left by the retirement of the shuttles—and to allow the space agency to focus on longer-range missions, such as sending rovers to Mars, NASA has said.
The hope is that NASA astronauts will soon be able to buy tickets to space in much the same way the rest of us buy airline tickets.
It's an exciting era, said David Brin, a science fiction writer who holds a Ph.D. in space science from the University of California, San Diego.
"Ever since Challenger blew up [in 1986], NASA's approach has been to redouble caution," added Brin, who applauds the risktaking ethos he sees among the private-shuttle contenders.
(See "Rare Pictures From the Dawn of NASA Spaceflight.")