WASHINGTON, June 10 (Reuters) - Boeing's ( BA ) Starliner

spacecraft scored a crucial achievement last week with the

delivery of two astronauts to the International Space Station,

but problems encountered along its journey in space and more

hurdles that lie ahead make the aerospace giant's goal toward

routine missions a distant prospect.

The CST-100 Starliner capsule's first crewed docking with

two astronauts to the International Space Station on Thursday

marked a long-sought safety demonstration for two audiences:

NASA, which wants a second U.S. spacecraft for rides to orbit,

and the nascent market for private astronaut missions that is

currently dominated by Elon Musk's SpaceX and its Crew Dragon

capsule.

But before Boeing ( BA ) can chip away at SpaceX's grip on

government and private orbital human spaceflight, its Starliner

has several more test objectives to reach.

"It's a critical step, because if they can't successfully

transport human beings to space, and safely return them, then

they haven't proven what they need to do to conduct any

missions," said Patricia Sanders, who until February was the

longtime chair of NASA's Aerospace Safety Advisory Panel.

The crew - veteran astronauts and test pilots Butch Wilmore

and Suni Williams - could return to Earth as soon as June 14 or

stay as long as 45 days, NASA officials said.

During Starliner's 24-hour trek to reach the space station

orbiting some 240 miles (386 km) above, the gumdrop-shaped

spacecraft sprang four helium leaks and had five onboard

thrusters go dead, delaying its ISS docking.

"Starliner made us work a little harder to get docked,"

NASA's commercial crew chief Steve Stich told a news conference

Thursday night.

But some achievements include Wilmore taking manual control

and testing steering, overall mission safety and the craft

autonomously docked at the station. Over the next several days,

Starliner will aim to show it can undock, maneuver more, then

safely return to Earth.

Still, the helium leaks and propulsion failures, while no

danger to astronauts, NASA officials said, are a nagging

concern.

Boeing ( BA ) first found a leak of helium - used to add pressure

to the thruster's propellants - while Starliner was on the

ground last month, and NASA officials deemed it low-risk for

flight. NASA officials said the thruster failures appeared

similar to those found on Starliner's 2022 uncrewed test to the

ISS.

"We don't quite understand why they're happening," Stich

said.

Boeing ( BA ) has said it plans to redesign valves on Starliner's

propulsion system after the company and NASA identified a flaw

in 2022. And the company is receiving $5.5 million from NASA to

study potentially redesigning Starliner's batteries, according

to federal contract records.

"If they had something that required a design change that

was going to be expensive and time-consuming, it may impact

their business decision for going forward," Sanders said.

Starliner development challenges already have cost Boeing ( BA )

roughly $1.5 billion.

Starliner redesigns could drag out if Boeing ( BA ) has to involve

its supply chain. In 2022, Boeing ( BA ) and its propulsion system

supplier, L3 Harris' Aerojet Rocketdyne, feuded for

months over which company was responsible for defects in

Starliner's propulsion system valves and who would pay for the

components' redesign, Reuters reported that year.

The company said it might implement that design change on a

later flight, while a temporary workaround is in place.

Musk has boasted SpaceX as having an edge in being more

vertically integrated.

A novel valve issue on SpaceX's Crew Dragon in 2019 caused

one of its spacecraft to explode during an uncrewed ground test

in 2019, prompting SpaceX to redesign the capsule's abort

propulsion system. The redesigned system flew its first crew

roughly a year later. And the company in 2021 redesigned Crew

Dragon's toilet in less than two months.

"Fixing a design flaw or even something of concern like a

battery is much, much easier in a vertically integrated

company," said Abhi Tripathi, former SpaceX Crew Dragon mission

director who was involved in the spacecraft's design changes.

It is not yet clear to NASA officials if the issues that

occurred during Starliner's first crewed mission would warrant

redesigns. NASA and Boeing ( BA ) will spend months reviewing mission

data and examining the flight problems to determine if Starliner

can be certified for routine flights.

"It's not a success until they safely return, and until we

understand the implications of any anomalies that occurred

during the mission," Sanders said.