After an intense competition, NASA announced contracts Friday totaling up to $900 million to be divvied up between three companies -- SpaceX, Boeing and Sierra Nevada -- to continue development of commercial manned spacecraft to ferry astronauts to and from the International Space Station.

Former shuttle astronaut Rex Walheim inside a SpaceX Dragon capsule.

While it is far from clear whether Congress will provide enough funding to keep all three companies in the mix, NASA Administrator Charlie Bolden, a former shuttle commander, said the program was critical to America's future in space.

"Today we are announcing another critical step towards launching our astronauts from U.S. soil on space systems built by American companies," Bolden said. "We have selected three companies to develop crew transportation capabilities as a fully integrated system and keep us on track to end the outsourcing of our human spaceflight program."

The Commercial Crew Integrated Capability Initiative -- CCiCap -- is the third round of post-shuttle contracts aimed at spurring private industry to develop a low-cost manned spacecraft to carry crews to and from the space station and end the agency's reliance on Russian Soyuz ferry flights that cost U.S. taxpayers more than $60 million a seat.

The new spacecraft also could be flown for purely commercial purposes with seats or entire flights booked by other governments or companies for research, Earth observation or yet-to-be-defined objectives if a commercial market emerges.

"This is a diverse and dynamic mix of companies, each with unique experiences and proven track records in the aerospace industries," Bolden said. "By keeping these three companies in the mix, we not only ensure competition, which is good for taxpayers, but we're also guaranteeing that we never find ourselves in the situation we're in today -- depending on a sole provider to get our crews to space."

Going into the latest round of contracts, four companies were executing initial spacecraft design studies -- SpaceX of Hawthorne, Calif., a fast-moving startup owned and operated by internet entrepreneur Elon Musk; Boeing Corp. of Houston, a company with extensive experience in the U.S. space program; Sierra Nevada of Louisville, Colo.; and Blue Origin of Kent, Wash., a secretive company owned by Amazon founder Jeff Bezos.

Three other companies were developing concepts through unfunded Space Act Agreements.

But on Friday, NASA narrowed the list of funded companies to three: SpaceX, Boeing and Sierra Nevada. Unlike the earlier design studies, the CCiCap contracts require the participants to develop an integrated solution that includes ground infrastructure, launch vehicles and spacecraft.

SpaceX was awarded a $440 million contract to continue development of a manned version of the company's Dragon cargo ship, which completed its first test flight to the International Space Station in May.

The SpaceX manned capsule will seat up to seven astronauts and rely on an upgraded version of the company's home-grown Falcon 9 rocket to reach orbit. Company engineers planning a rocket-assisted ground landing system. Initial test flights could come as early as 2015, depending on funding and engineering progress.

"This is a decisive milestone in human spaceflight and sets an exciting course for the next phase of American space exploration," SpaceX CEO Elon Musk said in a statement. "SpaceX, along with our partners at NASA, will continue to push the boundaries of space technology to develop the safest, most advanced crew vehicle ever flown

Boeing won a contract valued at $460 million to develop its CST-100 capsule, The spacecraft will seat up to seven astronauts and fly atop a United Launch Alliance Atlas 5 rocket. The CST-100 will make a parachute descent to a ground landing. Barring technical problems or budget issues, the first manned test flight is expected in 2016.

Sierra Nevada of was awarded a $212.5 million control to continue developing the winged "Dreamchaser," a small spaceplane originally developed by NASA as a space station lifeboat. The Dreamchaser lifting body will seat seven and launch stop an Atlas 5. Like the now-retired space shuttle, it would land on a runway at the end of a mission.

The CCiCAP contracts will run between now and May 31, 2014. While manned test flights could begin as early as 2015, agency officials say operational NASA flights to and from the space station will not begin until 2017 thanks to previous budget shortfalls.

"This is a diverse and dynamic mix of companies, each with unique experiences and proven track records in the aerospace industries," Bolden said. "By keeping these three companies in the mix, we not only ensure competition, which is good for taxpayers, but we're also guaranteeing that we never find ourselves in the situation we're in today -- depending on a sole provider to get our crews to space.

"For the next 21 months, these partners will perform tests and complete designs. Through this initiative, NASA will help the private sector design and develop the human spaceflight capability that could ultimately lead to the availability of human spaceflight services for both government and commercial customers."

In 2004, the Bush administration ordered NASA to complete the International Space Station and retire the shuttle by the end of 2010 to free up money for a new initiative to establish Antarctica-style bases on the moon. But the administration never fully funded its Constellation moon program and President Obama opted for a drastic change of course.

The moon program was canceled and NASA was ordered to take a two-pronged approach to manned spaceflight. The Orion capsule originally envisioned for Constellation flights to the moon would be used by NASA for eventual flights to a variety of deep space targets with the ultimate goal of reaching Mars in the 2030s.

The Obama administration initially deferred development of the heavy-lift rocket needed for deep space missions, but congressional space supporters successfully lobbied to accelerate the rocket's development. An initial test flight is planned for 2014.

NASA also was directed to fund a commercial initiative to develop new manned spacecraft to service the space station in low-Earth orbit. But congressional support of the commercial initiative has been decidedly mixed.

The administration asked for $800 million for commercial manned spaceflight in its fiscal 2012 budget request, but Congress approved just $400 million, a cut that pushed the first NASA flight to the station back one year to 2017.

This year, the administration requested $830 million in its fiscal 2013 budget. Early debate in the House called for limiting the scope of the contract to a single company but a compromise eventually was reached that would provide $500 million.

But the full $900 million needed for the three CCiCap contracts is required if all three companies are to proceed with development. If a budget including full funding is not approved, or if a continuing resolution is required because of a budget impasse, one of the companies likely would fall by the wayside.