NASA plans to use $US50 million of federal economic stimulus funds to seed development of a commercial passenger transportation service to space.
Aspiring spaceship operators have 45 days to submit proposals, which will be competitively evaluated. Awards for the programme are expected to be announced before the end of September.
The United States is retiring its fleet of space shuttles next year after seven more missions to complete construction of the $US100 billion International Space Station, which is in orbit about 360km above Earth.
After that, NASA plans to charter rides for astronauts to and from the station from Russia.
The agency is already spending $US500 million to help two US firms, Space Exploration Technologies and Orbital Sciences Corp, develop rockets and capsules to deliver cargo to the station.
NASA plans to hold a workshop in Houston on Thursday for companies interested in partnering to develop commercial passenger service to space. The competition is only open to US firms.
