CAPE CANAVERAL, Florida, June 1 (Reuters) - Boeing's ( BA )

new Starliner astronaut capsule is poised for launch on

Saturday in a much-delayed first crewed test flight, a milestone

in the beleaguered aerospace giant's aim to compete with Elon

Musk's SpaceX in the astronaut launch business.

The CST-100 Starliner with two astronauts aboard is due for

liftoff at 12:25 p.m. ET (1625 GMT) from NASA's Kennedy Space

Center in Florida, strapped to an Atlas V rocket from

Boeing-Lockheed Martin joint venture United Launch

Alliance (ULA).

A May 6 countdown was halted just two hours before launch

time over a faulty pressure valve on the Atlas rocket. A helium

leak and another issue subsequently were detected in Starliner's

propulsion system. All have been resolved, according to Boeing ( BA )

and NASA.

"This is a test flight, we know we're going to learn some

things," Boeing's ( BA ) commercial crew vice president Mark Nappi said

during a news conference on Friday.

The gumdrop-shaped capsule and its crew are bound for the

International Space Station (ISS), two years after the Starliner

completed its first test voyage to the orbital laboratory

without astronauts aboard.

Boeing ( BA ), with its commercial airplane operations staggering

from a series of crises involving its 737 MAX jetliners, needs a

win in space for its Starliner venture, already several years

behind schedule and more than $1.5 billion over budget.

The company is a longtime NASA contractor that has built

modules for the decades-old ISS and rockets designed to loft

astronauts toward the moon. But never before has it built its

own operational spacecraft, a feat complicated by years of

software issues, technical glitches and management shakeups on

the Starliner program.

While Boeing ( BA ) has struggled, SpaceX has become a dependable

taxi to orbit for the U.S. space agency, which is backing a new

generation of privately built spacecraft for flying astronauts

to low-Earth orbit and - under its ambitious Artemis program -

on to the moon and eventually Mars.

Starliner would compete head-to-head with SpaceX's Crew

Dragon capsule, which since 2020 has been NASA's only vehicle

for sending ISS crew members to orbit from U.S. soil. NASA has

long sought two U.S. rides to the station, in addition to the

joint astronaut flights it conducts with Russia's Soyuz rocket.

The inaugural crew for the seven-seat Starliner includes two

veteran NASA astronauts: Barry "Butch" Wilmore, 61, a retired

U.S. Navy captain and fighter pilot, and Sunita "Suni" Williams,

58, a former Navy helicopter test pilot with experience flying

more than 30 different aircraft.

They have spent a combined 500 days in space over the course

of two ISS missions each. Wilmore is the designated commander

for Saturday'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. The test

flight calls for Wilmore and Williams to practice maneuvering

the vehicle manually en route to the space station, where it

will remain docked for at least eight days before returning to

Earth.

If Boeing ( BA ) delays its Saturday launch attempt, the company

has backup launch opportunities on Sunday, Wednesday and

Thursday. And if it cannot make Thursday, some items on

Starliner and the rocket would need to be replaced or

replenished, spurring delays of weeks or potentially months

given conflicting schedules with other ULA missions and the ISS.

Saturday's flight marks the first crewed Atlas voyage to

space since earlier versions of the storied rocket dynasty first

sent U.S. astronauts, including John Glenn, to orbit during

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.

Wilmore and Williams are expected to remain at the space

station for about a week before riding the capsule back to Earth

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

Southwest - a first for a crewed NASA mission.