CAPE CANAVERAL, Florida, May 6 (Reuters) - Boeing Co's ( BA )

new Starliner astronaut capsule was poised for a Monday

night launch of its long-delayed inaugural crewed test flight to

the International Space Station, two years after its first

voyage without humans to the orbital laboratory.

The gumdrop-shaped CST-100 Starliner with two astronauts

aboard was due for liftoff at 10:34 p.m. (0234 GMT on Tuesday)

from NASA's Kennedy Space Center in Florida, carried atop an

Atlas V rocket furnished by the Boeing-Lockheed Martin

joint venture United Launch Alliance (ULA).

Riding aboard the Starliner, designed to carry up to seven

crew members, are veteran NASA astronauts Barry "Butch" Wilmore,

61, a retired U.S. Navy captain, and Sunita "Sunni" Williams,

58, a former Navy aviator and test pilot.

They have logged a combined 500 days in space over the

course of two missions each to the space station. Wilmore is the

designated commander for Monday's flight, with Williams in the

pilot seat.

Although Starliner is designed to fly autonomously, the crew

can assume control of the spacecraft if necessary, and the test

flight calls for Wilmore and Williams to practice maneuvering

the vehicle manually.

With forecasts calling for a 95% chance of favorable weather

conditions, ULA completed a launch readiness review on Friday

and gave a "go" to proceed toward countdown at Cape Canaveral.

The launch would mark the first crewed voyage to space using

an Atlas rocket since the storied family of Atlas launch

vehicles first sent astronauts, including John Glenn, on orbital

flights for NASA's Mercury program in the 1960s.

If all goes as planned, the capsule will arrive at the space

station after a flight of about 26 hours and dock with the

orbiting research outpost some 250 miles (400 km) above Earth

early on Wednesday.

Wilmore and Williams are expected to remain at the space

station for about a week before riding the Starliner back to

Earth for a parachute and airbag-assisted landing in the U.S.

Desert Southwest - a first for crewed NASA missions.

Success is vital before the Starliner gains final approval

to begin routine operational flights under NASA's commercial

crew program, providing the space agency its first alternative

to sending astronauts to low-Earth orbit from U.S. soil since

Elon Musk's SpaceX began doing so in 2020.

The test flight comes at an especially critical moment for

Boeing ( BA ), whose commercial aircraft unit has endured a succession

of safety crises in recent years.

Getting Starliner to this point has been a fraught process

for Boeing ( BA ), beset by years of development setbacks and more than

$1.5 billion in cost overruns for the aerospace giant on a $4.2

billion fixed-priced contract with NASA.

The first attempt by Boeing ( BA ) to send an uncrewed Starliner to

the space station in 2019 failed due to dozens of software and

engineering glitches. A second attempt in 2022 succeeded, paving

the way for Monday night's mission.

Depending on the outcome, Starliner is booked to fly at

least six more crewed missions to the space station for NASA.

At a pre-flight briefing with NASA officials on Friday,

Boeing ( BA ) vice president Mark Nappi said the company and the space

agency were "in total agreement" that the Starliner was ready to

fly with crew, adding, "we can't be more confident."

"Because it is a test flight, we give extra attention," NASA

chief Bill Nelson said. "That's why we put two test pilots on

board."