Boeing Starliner crewed mission postponed shortly before launch

WASHINGTON - The first crewed flight of Boeing's Starliner spaceship was dramatically called off around two hours before launch after a new safety issue was identified, pushing back a high-stakes test mission to the International Space Station.

Astronauts Butch Wilmore and Suni Williams were strapped in their seats preparing for liftoff when the call for a "scrub" came, in order to give engineers time to investigate unusual readings from an oxygen relief valve on the second stage of the rocket.

"Standing down on tonight's attempt to launch," tweeted NASA chief Bill Nelson. "As I've said before, @NASA's first priority is safety. We go when we're ready."

The next possible launch date comes on Tuesday night, but it wasn't immediately clear how big the problem was and if it could be resolved with the rocket still on the launchpad. NASA said it would hold a late-night press briefing to provide updates.

The mission has already faced years of delays and comes at a challenging time for Boeing, as a safety crisis engulfs the century-old manufacturer's commercial aviation division.

NASA is banking on a successful test for Starliner so it can certify a second commercial vehicle to carry crews to the ISS. 

Elon Musk's SpaceX achieved the feat with its Dragon capsule in 2020, ending a nearly decade-long dependence on Russian rockets following the end of the Space Shuttle program.

Clad in Boeing's bright blue spacesuits, the astronauts were helped out of the spaceship then boarded a van to leave the launch tower at Cape Canaveral Space Force Station, returning to their quarters.

Wilmore and Williams, both Navy-trained pilots and space program veterans, have each been to the ISS twice, travelling once on a shuttle and then aboard a Russian Soyuz vessel.