Sunita Williams' 3rd Mission to Space Called Off Hours Before Liftoff

Astronauts Butch Wilmore and Sunita Williams were strapped into their seats preparing for liftoff when the call for a "scrub" came, because engineers noticed audible buzzing from a liquid oxygen relief valve on the Atlas V rocket meant to propel the capsule into orbit.

In a late night press conference, Tory Bruno, president and CEO of United Launch Alliance (ULA) that built the rocket, told reporters the unusual vibrations were a sign the valve parts might be wearing down to the point of failure, but insisted "the crew was never in any danger."

Engineers will now work through the night to determine the degree of deterioration and decide whether to re-fit the same part on the launchpad, or wheel the rocket back to its assembly building to install a new valve, he added.

ULA, a Boeing-Lockheed Martin joint venture, should announce their decision by morning. The next possible launch date is Tuesday night, with more later this week and into next week.

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.