A Russian Soyuz spacecraft. NASA

NASA hopes to begin launching U.S. and partner astronauts to the space station aboard Boeing and SpaceX ferry craft in the 2017 timeframe, but agency managers expect to continue sending crew members up aboard Russian Soyuz spacecraft -- and Russian cosmonauts aloft aboard U.S. vehicles -- as a hedge against problems, like crew illness, that could force some station crew members to make an emergency return to Earth.

Without mixed crews, an illness could force everyone who came up with the sick crew member to depart aboard the vehicle that brought them to the station. If it was a U.S. or partner crew member, everyone who launched with that astronaut aboard a Boeing or SpaceX ferry craft would have to return to Earth, leaving the station in the hands of Russian cosmonauts who launched aboard a Soyuz spacecraft and who are not trained to operate NASA systems.

And vice versa.

As a result, NASA managers believe it makes long-range sense to launch one U.S. or partner astronaut aboard every Soyuz and one cosmonaut aboard every NASA commercial ferry ship. Nothing has been finalized, officials say, but the U.S. space agency does not expect either side to pay for seats on each other's spacecraft after Boeing and SpaceX begin operational flights.

"I wouldn't call it a barter for seats, it would be more of an operational understanding," Mike Suffredini, the space station program manager at the Johnson Space Center in Houston, told CBS News in an email early Tuesday. "We would not be buying seats from each other."

The International Space Station.

NASA

"An example would be if a crew member became incapacitated on orbit," Suffredini said. "In this scenario, the entire crew that flew up with the incapacitated crew member has to go home with that crew member to care for the (ill astronaut), fly the vehicle and because if someone stayed on ISS they would be without a rescue vehicle.

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NASA will keep using Russian Soyuz ferry craft