Cape Canaveral, Florida - The first crewed flight of Boeing's Starliner spaceship was dramatically called off Saturday with just under four minutes left on the launch countdown clock, for reasons that aren't yet entirely clear.

It was the second time the test mission to the International Space Station was scrubbed with the astronauts strapped in and ready to lift off – and yet another setback for the troubled program, which has already faced years of delays and safety concerns.

Astronauts Butch Wilmore and Suni Williams are waiting to be safely removed from the capsule.

Mission commander Wilmore had earlier given a short but rousing speech telling tens of thousands of people tuning into the live feed that "it's a great day to be proud of your nation."

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The former US Navy test pilots, who each have two spaceflights under their belts, were previously called back to quarantine after an aborted launch attempt on May 6 due to a faulty valve on the United Launch Alliance Atlas V rocket.

A backup date is available for Sunday, but it's not yet known whether the Starship will be ready to launch.

Starliner is poised to become the sixth type of US-built spaceship to fly NASA astronauts, following the Mercury, Gemini, and Apollo programs in the 1960s and 1970s, the Space Shuttle from 1981 to 2011, and SpaceX's Crew Dragon from 2020.

Boeing, with its 100-year history, was heavily favored over its then-upstart competitor, but its program fell badly behind amid embarrassing setbacks that mirrored the myriad problems afflicting its commercial airline division.

These ranged from a software bug that put the spaceship on a bad trajectory on its first uncrewed test, to the discovery that the cabin was filled with flammable electrical tape after the second.

While teams were working to replace the faulty valve that postponed the previous launch attempt, a small helium leak located in one of the spacecraft's thrusters came to light.

But rather than replace the seal, which would require taking Starliner apart in its factory, NASA and Boeing officials then declared it safe enough to fly as is.