Boeing has won the lion’s share of the $6.8 billion NASA wants to spend on its next two human spaceflight vehicles over the next five years, drawing a $4.2 billion share for its CST-100 capsule versus the $2.6 billion SpaceX will get to finish developing and fly the crew version of its Dragon cargo vehicle.

Both will work to the same requirements, and both award amounts were based on what the companies said they would need to do the work of getting their vehicles certified to fly four-member crews to the International Space Station, according to Kathy Lueders, NASA’s commercial crew program manager.

Losing out in the competition to return human space launch to U.S. soil was Sierra Nevada Corp., the third company that had received NASA seed money under the agency’s commercial crew-vehicle development program. In picking Boeing and SpaceX, NASA went with traditional capsule designs over the reusable lifting-body approach Sierra Nevada advanced with its Dream Chaser vehicle.

Boeing’s CST-100 is an aluminum capsule with an ablative heat shield, designed to return to Earth on dry land under parachutes, with airbags deploying to soften the final touchdown. It is intended to lift off on the United Launch Alliance Atlas V.

The crew-version Dragon is an upgrade of the Dragon already delivering cargo to the space station under NASA’s Commercial Resupply Services contract, which includes returns after reentry with scientific samples and other "down-mass" cargo under parachute to a splashdown in the Pacific Ocean off the coast of California. Both Dragon variants are designed to be launched on the company’s Falcon 9 rocket.

Lueders declined to specify factors leading to the downselect, saying the agency has yet to brief the companies who bid for the work. Under their contracts, Boeing and SpaceX each must meet five milestones, including a demonstration flight to the ISS with at least one NASA astronaut on board, to qualify for between two and six operational missions.

"Our specialist teams have watched the development of these new spacecraft during earlier development phases, and are confident they will meet the demands of these important missions," said NASA Administrator Charles Bolden, who announced the selections at a Kennedy Space Center press conference Tuesday afternoon. "We are also confident they will be safe for NASA astronauts. To achieve NASA certification in 2017, they must meet the same rigorous safety standards we had for the space shuttle program."

NASA said the two companies will work with the agency to complete the Certification Baseline Review within 90 days of the contracts’ start dates. The agency has a goal of completing certification, including the test flights, by the end of 2017, and will award task orders for "post-certification missions" for as long as five years after the effective date of the contract. Lueders said the two contracts also have extension provisions, and the agency will continue to work with Sierra Nevada, Blue Origin and other commercial crew Space Act Agreement partners should they desire to continue their developments for possible future missions.

Bolden and other NASA human spaceflight officials have repeatedly argued for a two-vehicle approach, both for redundancy and to hold down costs through competition. However, Congress has proved skeptical of that approach, largely because it would cost more. The agency has requested $848.3 million in fiscal 2015 for commercial spaceflight, although it is more likely to receive funding at the $646 million fiscal 2014 level under the continuing resolution lawmakers are expected to approve before recessing for the November elections.

"As we have said, in order for us to get to 2017 what we really need is for the Congress to support the president’s request," Bolden said. " We are confident that given where we are right now with the 2014 budget and its out-run, we can make the 2017 launch date. But that again depends on Congress fully funding the budget as requested by the president."