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 launch pad. 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 programme.
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 US Navy-trained pilots and space programme veterans, have each been to the ISS twice, travelling once on a shuttle and then aboard a Russian Soyuz vessel.
When it launches, Starliner will be propelled into orbit by an Atlas V rocket made by United Launch Alliance, a Boeing-Lockheed Martin joint venture. The crew will then take the helm, piloting the craft manually to test its capabilities.
The gumdrop-shaped capsule with a cabin about as roomy as an SUV is then set to rendezvous with the ISS for a weeklong stay.
Williams and Wilmore will conduct a series of tests to verify Starliner’s functionality before returning to Earth for a parachute-assisted landing in the western United States.
A successful mission would help dispel the bitter taste left by numerous setbacks in the Starliner programme.
In 2019, during a first uncrewed test flight, software defects meant the capsule was not placed on the right trajectory and returned without reaching the ISS. “Ground intervention prevented loss of vehicle,” said Nasa in the aftermath, chiding Boeing for inadequate safety checks.
Then in 2021, with the rocket on the launch pad for a new flight, blocked valves forced another postponement.
The vessel finally reached the ISS in May 2022 in a non-crewed launch. But other problems that came to light – including weak parachutes and flammable tape in the cabin that needed to be removed – caused further delays to the crewed test flight, necessary for the capsule to be certified for Nasa use on regular ISS missions.
SpaceX’s Dragon capsule joined that exclusive club four years ago, following the Mercury, Gemini, Apollo and Space Shuttle programmes.
In 2014, the agency awarded fixed-price contracts of US$4.2 billion to Boeing and US$2.6 billion to SpaceX to develop the capsules under its Commercial Crew Programme.
This marked a shift in Nasa’s approach from owning space flight hardware to instead paying private partners for their services as the primary customer.
SpaceX CEO Elon Musk took a swipe at Boeing, gloating that his company “finished 4 years sooner” despite receiving a smaller contract. He attributed Boeing’s delay to “too many non-technical managers” in a post on X.
Once Starliner is fully operational, Nasa hopes to alternate between SpaceX and Boeing vessels to taxi humans to the ISS.
Even though the orbital lab is due to be mothballed in 2030, both Starliner and Dragon could be used for future private space stations that several companies are developing.