CAPE CANAVERAL, Florida, June 1 (Reuters) - The launch

countdown for Boeing's ( BA ) new Starliner space capsule on its

inaugural crewed test flight was halted on Saturday, postponing

the mission for at least 24 hours.

The CST-200 Starliner's first voyage carrying two astronauts

to the International Space Station (ISS) has been highly

anticipated and much-delayed as Boeing ( BA ) scrambles to gain a

greater share of lucrative NASA business now dominated by Elon

Musk's SpaceX.

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

Starliner to the space station in 2019 failed due to software

and engineering glitches. A second try in 2022 succeeded, paving

the way for efforts at getting the first crewed test mission off

the ground.

A May 6 countdown was halted just two hours before

launch time over a faulty pressure valve on the Atlas upper

stage, followed by weeks of further delays caused by other

engineering problems, since resolved, on the Starliner itself.

The gumdrop-shaped capsule had stood poised for blastoff

from NASA's Kennedy Space Center in Florida atop an Atlas V

rocket furnished by United Launch Alliance, a Boeing-Lockheed

Martin joint venture.

The postponement was announced during a live NASA webcast.

Launch forecasts on Saturday had called for a 90% chance

of favorable weather conditions, and ULA said on social media

earlier in the day that all systems were "go" for an on-time

liftoff.

Boeing ( BA ), whose commercial plane operations are in disarray

after several sequential crises, badly needs a win in space for

its Starliner venture, a program several years behind schedule

with more than $1.5 billion in cost overruns.

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

taxi to orbit for NASA, which is backing a new generation of

privately built spacecraft that can ferry astronauts to ISS and

in the future - under its ambitious Artemis program - 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 to orbit from U.S. soil.

The flight 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.

Once launched, the capsule is expected to 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.

Plans call for the two astronauts 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.

The test flight comes at an especially critical moment for

Boeing ( BA ). Its airplane business is dealing with fallout from a

midair blowout of a cabin panel door plug on a nearly new 737

MAX 9 in January, as well as previous deadly crashes of two 737

MAX jets.

Getting Starliner to this point has been a fraught process

for Boeing ( BA ) under a $4.2 billion fixed-priced contract with NASA

that has since swelled to roughly $4.5 billion, according to a

Reuters review of contract changes since it was awarded in 2014.

The space agency wants the redundancy of having two

different U.S. rides to the ISS, which is expected to retire

around 2030. NASA is encouraging private development of new

space stations that could replace the ISS after its retirement,

potentially giving Starliner new destinations.

Depending on the outcome of the first crewed test flight,

Starliner is booked to fly at least six more crewed missions to

the space station for NASA.