June 21 (Reuters) -

Boeing Starliner's return to Earth from the International

Space Station with its first crew of astronauts has been

postponed, NASA said on Friday.

NASA did not provide a new date, raising questions about

when the mission's two astronauts will return as more testing

and technical issues have created more delays.

The return to earth was previously scheduled for June 26

The crew of U.S. astronauts, Butch Wilmore and Suni

Williams, lifted off June 5 as a final demonstration to obtain

routine flight certification from NASA.

The crewed test of the spacecraft, which has been test-flown

to space twice since 2019 without humans aboard, has encountered

five failures of its 28 maneuvering thrusters, five leaks of

helium that is meant to pressurize those thrusters, and a

slow-moving propellant valve that signaled unfixed issues from

the past.

The issues and the additional tests NASA and Boeing ( BA ) have had

to do call into question when exactly Starliner will be able to

fly its crew home, and add to a list of broader problems Boeing ( BA )

faces with its Starliner program. The company has spent $1.5

billion in cost overruns on top of its $4.5 billion NASA

development contract.

NASA wants Starliner to become a second U.S. spacecraft

capable of ferrying its astronauts to and from the ISS alongside

SpaceX's Crew Dragon, which has been the agency's primary ride

since 2020. Boeing's ( BA ) Starliner program has struggled with

software glitches, design problems and subcontractor disputes

for years.

When Starliner arrived in the space station's vicinity to

dock on June 6, the five thruster failures prohibited the

spacecraft from making a close approach until Boeing ( BA ) could

implement a fix. The company rewrote software and tweaked some

procedures to revive four of them and proceed with a docking.

Starliner's undocking and return to Earth represent the

spacecraft's most complicated phases of its test mission. NASA

officials have said they want to better understand the cause of

the thruster failures, valve issue and helium leaks before

Starliner embarks on its roughly six-hour return journey.

While just one thruster remains dead in Starliner's current

flight, Boeing ( BA ) encountered four thruster problems during the

capsule's uncrewed return from space in 2022.

According to flight rules established jointly by Boeing ( BA ) and

NASA, Starliner's maneuvering thrusters must, at a minimum,

allow for "6-degrees of freedom of control," and each have one

backup thruster, a NASA spokesperson told Reuters.

That could mean at least 12 of the 28 thrusters - most of

which are backups - are required for a safe flight, or

potentially fewer, as long as the remaining thrusters have one

backup and can work together in a way that doesn't restrict

Starliner's freedom of movement in space.