WASHINGTON, June 24 (Reuters) - Problems with Boeing's ( BA )

Starliner capsule, still docked at the International Space

Station (ISS), have upended the original plans for its return of

its two astronauts to Earth, as last-minute fixes and tests draw

out a mission crucial to the future of Boeing's ( BA ) space

division.

NASA has rescheduled the planned return three times, and now

has no date set for it. Since its June 5 liftoff, the capsule

has had five helium leaks, five maneuvering thrusters go dead

and a propellant valve fail to close completely, prompting the

crew in space and mission managers in Houston to spend more time

than expected pursuing fixes mid-mission.

Here is an explanation of potential paths forward for

Starliner and its veteran NASA astronauts, Barry "Butch" Wilmore

and Sunita "Suni" Williams.

THE CURRENT SITUATION

Starliner can stay docked at the ISS for up to 45 days,

according to comments by NASA's commercial crew manager Steve

Stich to reporters. But if absolutely necessary, such as if more

problems arise that mission officials cannot fix in time, it

could stay docked for up to 72 days, relying on various backup

systems, according to a person familiar with flight planning.

Internally at NASA, Starliner's latest targeted return date

is July 6, according to this source, who spoke on condition of

anonymity. Such a return date would mean that the mission,

originally planned for eight days, instead would last a month.

Starliner's expendable propulsion system is part of the

craft's "service module." The current problems center on this

system, which is needed to back the capsule away from the ISS

and position it to dive through Earth's atmosphere. Many of

Starliner's thrusters have overheated when fired, and the leaks

of helium - used to pressurize the thrusters - appear to be

connected to how frequently they are used, according to Stich.

Stich said recent test-firings of the thrusters while

Starliner remains docked gave mission teams confidence in a safe

return, though tests and reviews are ongoing. The mission

management team, made up of NASA and Boeing ( BA ) personnel, is

scrutinizing data on the propulsion issues, running simulations

in Houston and considering how to fix them, such as by updating

software or changing how the hardware is used.

Once NASA officials give the team a go-ahead for a return,

Starliner's thrusters would be used to undock the capsule from

the ISS and begin a roughly six-hour journey home, gradually

tightening its orbit before plunging into Earth's atmosphere for

a landing, assisted by parachutes and airbags, at one of several

potential locations in the southwestern United States.

This is Starliner's first mission to orbit carrying

astronauts - the final test needed before NASA can certify it as

the U.S. space agency's second ride to the ISS. It would join

SpaceX's Crew Dragon, which has dominated the government and

nascent private markets for human spaceflight amid Starliner's

years-long delays.

IF THE UNEXPECTED HAPPENS

Even with the propulsion system issues, NASA has said

Starliner still would be capable of returning the astronauts to

Earth if absolutely necessary - that is, if the capsule must

serve as an escape pod from the ISS in an emergency or if any of

Starliner's perishable items - such as its solar panels - show

signs of expiring earlier than planned.

Unlike Starliner's current mission, NASA did not set a

scheduled return date for Crew Dragon's first mission carrying

astronauts in 2020. That mission ultimately lasted 62 days

because the astronauts needed to help out on ISS maintenance

because the space station was short-staffed at the time.

IF STARLINER CANNOT BE USED

If Starliner is deemed incapable of safely returning Wilmore

and Williams to Earth, one option would be sending them home

aboard Crew Dragon, which ferried four astronauts to the station

in March and is able to fit more people in an emergency.

That scenario, considered unlikely, would undoubtedly be

embarrassing for Boeing ( BA ). But NASA and Boeing ( BA ) officials, as well

as engineers familiar with the program, told Reuters nothing

about Starliner's current problems indicates this would be

needed.

In such a scenario, Starliner's fate would depend on various

factors including the extent of its technical issues.

The last time a NASA astronaut needed an alternative ride

home came in 2022, when Russia's Soyuz capsule sprang a coolant

leak after delivering to the station two cosmonauts and American

astronaut Frank Rubio.

NASA had considered Crew Dragon as an alternative ride home

for Rubio but he eventually used an empty Soyuz capsule that

Russia launched as a rescue craft. Rubio's mission was extended

from six months to a bit more than a year - 371 days - a

record-breaking duration for an American in space.