NASA plans to retire the International Space Station by the end of this decade, so the U.S. space agency is turning to private companies to build new space stations in orbit – and expects to save more than $1 billion annually as a result.

NASA earlier this year unveiled the Commercial LEO Destinations project, with plans to award up to $400 million in total contracts to as many as four companies to begin development of private space stations.

In response to NASA’s request, its director of commercial spaceflight, Phil McAlister, told CNBC that the agency “received roughly about a dozen proposals” from a variety of companies for contracts under the project.