The flight -- a final, crewed test before the capsule begins regular runs to the orbiting outpost -- comes at a critical time for Boeing, a century-old conglomerate whose passenger jet division has been engulfed in a safety crisis.

The stakes are also high for NASA. Having a second reliable option for rides to the space station (ISS) in addition to SpaceX's Dragon vehicles has been an agency priority ever since the last space shuttle flew in 2011, leaving the United States temporarily reliant on Russian Soyuz rockets.

Astronauts Butch Wilmore and Suni Williams are set to take off from Cape Canaveral Space Force Station at 10:34 pm Monday (0234 GMT Tuesday). Weather conditions are 95 percent favorable for launch.

Starliner will be propelled into orbit by an Atlas V rocket made by United Launch Alliance, a Boeing-Lockheed Martin joint venture. Once in space, the crew will take over piloting the craft.

The astronauts, both Navy-trained space program veterans, have each been to the ISS twice, traveling once on a shuttle and then aboard a Russian Soyuz vessel. "It's going to be like going back home," Williams said.

"(Safe) travels, star sailors," NASA chief Bill Nelson tweeted. "You are the pride of our great nation."

- Hiccups expected -

Starliner is scheduled to rendezvous with the ISS at about 0500 GMT Wednesday, kicking off a week-long stay. Tests will be performed to check if the capsule is working properly, and then Williams and Wilmore will reboard it to return home.

A successful mission would help dispel the bitter taste left by numerous setbacks in the Starliner program.

In 2019, during a first uncrewed test flight, two 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 launchpad for a new flight, blocked valves forced another postponement.

The empty vessel finally reached the ISS in May 2022. But problems since then -- 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.

NASA associate administrator Jim Free had predicted the mission would not be hiccup-free.

"We certainly have some unknowns in this mission, things we expect to learn, being a test mission," Free said, underscoring that Starliner is poised to become just the sixth type of US spaceship ever to carry NASA astronauts.

- Exclusive club -

SpaceX's Dragon capsule joined that exclusive club in 2020, following the Mercury, Gemini, Apollo and Space Shuttle programs.

In 2014, the agency awarded fixed-price contracts of $4.2 billion to Boeing and $2.6 billion to SpaceX to develop the capsules under its Commercial Crew Program.

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.

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