The companies' designs, like this "Dream Chaser" craft from Sierra Nevada Corp. (SNC), are widely varied. Courtesy: SNC.
Months after the first commercial cargo mission to the International Space Station, NASA has given the green light to three more companies to develop ways to get American astronauts into space from American soil.
The $10 million contracts were awarded Monday to Boeing, Sierra Nevada and SpaceX.
NASA says the end goal will be to "enable future certification of spaceflight as safe to carry humans to the International Space Station."
NASA used to rely on its space shuttle program to ferry astronauts to the I.S.S. Since the fleet's retirement last year, NASA astronauts have launched from the Russian Space Agency's Baikonur Cosmodrome, in Kazakhstan.
NASA has turned to the private sector to develop new ways to reach the station from the United States.
SpaceX, one of the companies awarded the new contracts, has already developed and successfully launched an unmanned space capsule, which docked with the ISS a few months ago as the first of several resupply missions.
The new contracts, NASA says, will require Boeing, SpaceX and Sierra Nevada to certify their planned space vehicles to meet NASA safety standards for human spaceflight.
The next phase, in 2014, will be an open competition that will hopefully end with development and testing, ahead of manned demonstration flights to the station.