Updated at 6:16 p.m. EDT

WASHINGTON — NASA awarded contracts worth $6.8 billion to Boeing and SpaceX on Sept. 16 to develop commercial crew transportation systems, culminating a long and sometimes controversial selection process.

Under the Commercial Crew Transportation Capability (CCtCap) contracts, the two companies will continue development of spacecraft capable of transporting NASA astronauts to and from the International Space Station as early as 2017, ending the agency’s dependence on Russian Soyuz spacecraft.

The contracts cover the development and certification of the spacecraft, including at least one test flight with both NASA and commercial crewmembers on board. The awards also fund between two and six operational flights to the ISS, each carrying four astronauts, once NASA certifies each company’s vehicle. Unlike previous phases of NASA’s commercial crew program, which used funded Space Act Agreements that provided greater flexibility, the CCtCap awards are fixed-price contracts.

“This was not an easy choice,” NASA Administrator Charles Bolden said at the Sept. 16 announcement at the Kennedy Space Center, “but this is the best choice for NASA and the nation.”

Boeing will receive $4.2 billion to build the CST-100 spacecraft, which it has been working on since the initial phases of NASA’s commercial crew program in 2010. The spacecraft will be launched on a United Launch Alliance Atlas 5 rocket.

“Boeing has been part of every American human space flight program, and we’re honored that NASA has chosen us to continue that legacy,” John Elbon, Boeing vice president and general manager for space exploration said in a company press release. “The CST-100 offers NASA the most cost-effective, safe and innovative solution to U.S.-based access to low-Earth orbit.”

SpaceX will receive $2.6 billion to build its Dragon V2 spacecraft, an upgraded version of the Dragon spacecraft currently used to transport cargo to and from the ISS. Dragon V2 will launch on the company’s Falcon 9 v1.1 rocket.

“SpaceX is deeply honored by the trust NASA has placed in us,” SpaceX Chief Executive and chief designer Elon Musk said in a statement provided by the company. “We welcome today’s decision and the mission it advances with gratitude and seriousness of purpose.”

The losing major competitor, Sierra Nevada, has sought funding to continue work on its Dream Chaser spacecraft, a lifting body vehicle launched atop an Atlas 5 but one that, unlike the CST-100 and Dragon capsules, glides to a runway landing. Company officials previously indicated that they would consider continuing development of Dream Chaser for non-NASA customers if not selected for a CCtCap contract.

Sierra Nevada, in a statement, said it was “disappointed” in the decision but would wait until a debrief with NASA before announcing its future plans. “When this process is complete and after a thorough evaluation, SNC will elaborate further on its future options” regarding both the CCtCap contract decision and the future of Dream Chaser.

NASA, in the press conference and a teleconference with reporters, declined to go into details about why NASA chose those Boeing and SpaceX, or other details of the competition, including the number of proposals it received. Kathy Lueders, NASA commercial crew program manager, said NASA would release more details about the selection, including a source selection statement, at an “appropriate” but unspecified later date.

Lueders also declined to go into detail about the contract amounts, particularly why Boeing received $1.6 billion more than SpaceX. “We basically awarded based on the proposals that we were given,” she said. “Both contracts have the same requirements. The companies proposed the value within which they were able to do the work, and the government accepted that.”

She added that while the goal is to have the companies’ vehicles certified to carry NASA astronauts to the ISS by the end of 2017, safety considerations would override that schedule goal. “We currently have plans and credible schedules to get certified by 2017,” she said of the proposals from Boeing and SpaceX. “We’re not going to sacrifice crew safety for that goal.”

The winning companies will be free to use their vehicles for commercial applications beyond transporting NASA astronauts to the ISS. Boeing has previously announced partnerships with commercial space tourism company Space Adventures and commercial orbital habitat developer Bigelow Aerospace Corp. to use Boeing CST-100 commercially.

All three companies had been involved in earlier phases of the program. Under the previous phase, Commercial Crew Integrated Capability (CCiCap), Boeing received $460 million, SpaceX $440 million, and Sierra Nevada $212.5 million in funded Space Act Agreements awarded in August 2012. Boeing completed the milestones in its CCiCap award in August, while Sierra Nevada and SpaceX are expected to finish their remaining CCiCap milestones in the next few months.

The three companies also received smaller awards in the Commercial Crew Development Phase 2 program, again using funded Space Act Agreements, in April 2011. Boeing and Sierra Nevada were also among the five companies that received the initial Commercial Crew Development awards in early 2010, using $50 million in stimulus funding provided to NASA as part of the American Recovery and Reinvestment Act of 2009.

NASA issued the request for proposals for CCtCap in November 2013, with proposals due to the agency in January. NASA had been under pressure from some in Congress to select a single company, believing that would accelerate the date a crew vehicle would be ready. NASA, though, had argued for selecting at least two companies in order to maintain competition.

Initial congressional reaction, though, was positive to the CCtCap awards. “I congratulate Boeing and SpaceX on their achievements in the Commercial Crew Program,” Rep. Lamar Smith (R-Tex.), chairman of the House Science Committee, said in a statement. He added, though, that the winning companies “have a responsibility to the U.S. taxpayers who are making considerable contributions to the development of these commercial space capabilities.”

Dan Leone contributed to this story from Washington. 

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...