CAPE CANAVERAL, Florida, May 6 (Reuters) - The

long-awaited first crewed test flight of Boeing's ( BA )

Starliner spacecraft was called off for at least 24 hours over a

technical glitch with the Atlas V rocket that was being readied

to launch the new astronaut capsule to orbit on Monday night.

The CST-100 Starliner's inaugural voyage carrying astronauts

to the International Space Station (ISS) has been highly

anticipated and much-delayed as Boeing ( BA ) scrambles to compete with

Elon Musk's SpaceX for a greater share of lucrative NASA

business.

It comes two years after the gumdrop-shaped capsule

completed its first test flight to the orbital laboratory

without humans aboard. The Starliner's first uncrewed flight to

the ISS in 2019 ended in failure.

Its latest flight was scrubbed with less than two hours left

in the countdown as the capsule 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, attributed to an issue with a valve in the

Atlas rocket's second stage, was announced during a live NASA

webcast.

It was not immediately clear how long the issue would

take to address, but the next available launch windows for the

mission are Tuesday, Thursday and Friday nights.

The two-member crew - NASA astronauts Barry "Butch" Wilmore,

61, and Sunita "Suni" Williams, 58 - had been strapped into

their seats aboard the spacecraft for about an hour before

launch activities were suspended.

They were subsequently assisted safely out of the capsule by

technicians and whisked away from the launch complex in a van to

await a second flight attempt once the issue has been resolved.

It is not uncommon in the space industry for countdowns to

be halted at the 11th hour and for launches to be postponed for

days or weeks, even when seemingly minor malfunctions or unusual

sensor readings are detected, especially in new spacecraft

flying humans for the first time.

Boeing ( BA ) faces intense public scrutiny of all its

activities after its commercial airplane operations have been

staggered by several

crises

, including the mid-air blowout of a plane door plug in

January. The company has been eager to get its Starliner space

venture off the ground to show signs of success and redeem a

program 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 its astronauts and

other customers to the ISS and, under the space agency's more

ambitious Artemis program, to the moon and eventually Mars.

Though Boeing ( BA ) has been relatively mute about its plans to

sell commercial Starliner flights, the spacecraft 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.

SEASONED TEST FLIGHT CREW

Selected to ride aboard Starliner for its first crewed

flight were two NASA veterans who have logged a combined 500

days in space over the course of two previous 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

astronauts can assume control of the spacecraft if necessary.

The test flight calls for Wilmore and Williams to practice

maneuvering the vehicle manually while en route to the ISS.

Ironically, the flight would mark the first crewed voyage to

space using an Atlas rocket since the storied series of launch

vehicles first sent astronauts, including John Glenn, on orbital

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

Once launched, 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. A resident

ISS crew, currently comprising four U.S. astronauts and three

Russian cosmonauts, will be there to greet them.

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 - the first time such a system has been used

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 ), beset by years of development setbacks and more than

$1.5 billion in charges for the aerospace giant on a $4.2

billion fixed-priced contract with NASA.

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 forthcoming flight test,

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

the space station for NASA.