Boeing and the Space Exploration Technologies Corporation are the winners in the competition to carry American astronauts to the International Space Station, NASA announced on Tuesday.

The awards reflect a fundamental shift in NASA’s human spaceflight program, relying on private companies rather than the traditional hands-on approach, in which the space agency designed and operated the spacecraft.

The first flights could take off as soon as 2017.

“We have credible plans for both companies to get there by that period of time,” Kathryn Lueders, the manager for NASA’s commercial crew program, said during a news conference on Tuesday. “We will not sacrifice crew safety for that goal.”

Boeing received a $4.2 billion contract. Space Exploration Technologies — better known as SpaceX, of Hawthorne, Calif. — received a $2.6 billion contract.