Nasa has issued a request for proposals for private companies to develop and build crew transportation systems to help the USA regain the capability to launch astronauts from own soil.
The agency wants to be able to purchase rides on a commercial basis before the end of 2017 to fly four crewmembers to and from the International Space Station (ISS) about every six months.
The new solicitation asks for proposals for final design, development, test, evaluation and certification of a human space transportation system, including ground operations, launch, orbital operations, return to Earth and landing. Nasa intends to award one or two Commercial Crew contracts next summer and would like to perform test flights in 2016.
The Commercial Crew Program follows the successful space cargo delivery program that has already provided Nasa with two privately operated cargo delivery vehicles capable of servicing the ISS. SpaceX has already conducted three missions to the orbital outpost, including one test flight and two cargo runs, while Orbital Sciences completed its test flight in September and is preparing its first resupply mission in December.
The USA has been dependent on Russia to ferry astronauts to and from the ISS since the end of the Space Shuttle Program in 2011. The three space shuttles were laid off due to high operation costs and safety concerns.
Rather than designing a modernised space shuttle and hiring contractors to build it, Nasa decided to get involved with the private industry, offering money, technical advice and oversight, while focusing itself on development of a system for deep space exploration.
Nasa's Space Launch System, consisting of the Orion capsule and a heavy-lift rocket, which is currently under development, is expected to become the most powerful space launch system ever developed.
Since 2011, SpaceX, Boeing and Sierra Nevada have worked with Nasa on design and development of commercial space taxis to fly astronauts to and from the low Earth orbit. Since the shuttles' retirement, only Russia has the capability to transport humans and charges Nasa more than $60per an astronaut ferried to the ISS.
Apart from the US government, several privately operated companies with ambitions to launch space hotels start orbital space tourism in the near future, including Bigelow Aerospace, have already expressed their interest in purchasing private flights.
The Obama administration is requesting $821m (£509m) for Nasa's Commercial Crew program for the fiscal year that began 1 October.
However, the Congress has not yet passed the 2014 budget. The Senate is proposing $775m for Commercial Crew while the House of Representatives wants to reduce the funding to $500m.