The United States has been without a human space transportation system since 2011 when NASA retired its three-ship shuttle fleet due to high operating costs and fundamental safety questions.
NASA's so-called Commercial Crew program is intended to address both cost and safety concerns, as well as return the capability to fly people to space from U.S. 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 station 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.
Rather than designing a replacement space shuttle and hiring contractors to build it, NASA decided to partner with industry, offering money, technical advice and oversight.
A precursor programme for cargo ships spawned two new supply lines to the International Space Station, a $US100 billion research outpost that flies about 400km above Earth.
So far, privately owned Space Exploration Technologies, or SpaceX, has made one test flight and two cargo runs to the station. Orbital Sciences Corp. completed its test flight in September and is preparing its first resupply mission in December.
NASA contributed about $US800 million toward the development of SpaceX's Falcon 9 rocket and Dragon cargo capsule and for Orbital Sciences' Antares rocket and Cygnus capsule. The companies also developed launch sites in Florida and Virginia, respectively, ground control stations and support services. Both firms also now offer orbital launch services commercially.
NASA followed up the development work with contracts worth a combined $US3.5 billion for SpaceX and Orbital Sciences to fly cargo to the station.
Since 2011, SpaceX, Boeing and privately owned Sierra Nevada Corp. have been NASA's partners in a sister programme to develop commercial space taxis to fly astronauts to and from the station. Since the shuttles' retirement, only Russia has the spaceships to ferry station crew members, a service that costs NASA more than $US60 million per person.
FULL FUNDING URGED
The Obama administration is requesting $US821 million for NASA's Commercial Crew programme for the fiscal year that began on October 1.
Congress has not yet passed a 2014 budget.
The Senate is proposing $US775 million for Commercial Crew; the House wants to cap the programme at $US500 million.
"It's now critically important to get full funding from Congress to keep us on track to begin these launches in 2017," NASA administrator Charles Bolden told reporters last week.
NASA on Tuesday issued what is expected to be the last step in the programme, with the goal of having test flights in 2016 and an operational system before the end of 2017, documents posted on NASA's procurement website show.
In addition to U.S. government business, privately owned Bigelow Aerospace, among others, intends to purchase flight services to ferry researchers, tourists and other paying passengers to planned orbital habitats.
NASA intends to award one or two Commercial Crew contracts next summer.
