Enlarge / Despite her smiles here, NASA's commercial crew program manager has concerns about schedules for Boeing and SpaceX. (credit: NASA)

Publicly, both Boeing and SpaceX maintain that they will fly demonstration missions by the end of this year that carry astronauts to the International Space Station. This would put them on course to become certified for "operational" missions to the station in early 2019, to ensure NASA's access to the orbiting laboratory.

On Wednesday, during a congressional hearing, representatives from both companies reiterated this position. "We have high confidence in our plan," Boeing's commercial crew program manager, John Mulholland, said. SpaceX Vice President Hans Koenigsmann said his company would be ready, too.

However their testimony before the US House Subcommittee on Space was undercut by the release of a report Wednesday by the US Government Accountability Office. The lead author of that report, Christina Chaplain, told Congress during the same hearing that she anticipated these certification dates would be much later. For SpaceX, operational flights to the station were unlikely before December, 2019, and Boeing unlikely before February, 2020, Chaplain said.

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