NASA has awarded Boeing Co. a contract worth as much as $4.2 billion and rival Space Exploration Technologies Corp. a separate pact valued at up to $2.6 billion to develop, test and fly space taxis to carry U.S. astronauts into orbit.

The American-built rockets and spacecraft would replace the Russian systems that currently provide the only access to the international space station. The space taxi is slated to transport its first astronaut to the orbiting laboratory by 2017.

Tuesday's long-awaited announcement by the National Aeronautics and Space Administration marks a milestone for commercial space ventures, partly because both companies will own and operate the equipment while experiencing significantly less day-to-day federal oversight than previous manned space programs.

The final value of the contracts, which likely will run past the end of the decade, depends on each of the contractors meeting specified timetables and obtaining NASA certification of their rockets and capsules for safety and reliability. Assuming the companies win those approvals, the contracts will cover as many as a dozen missions overall to shuttle astronauts and limited amounts of cargo to and from the space station.

The awards highlight the challenges of stretching limited federal dollars—and trying to meet aggressive schedules—while supporting two separate, full-fledged contractor teams.