WASHINGTON, Aug 24 (Reuters) - Two NASA astronauts who

flew to the International Space Station in June aboard Boeing's ( BA )

faulty Starliner capsule will need to return to Earth on

a SpaceX vehicle early next year, NASA said on Saturday, deeming

issues with Starliner's propulsion system too risky to carry its

first crew home.

The agency's decision, tapping Boeing's ( BA ) top space rival to

return the astronauts, is one of NASA's most consequential in

years. Boeing ( BA ) had hoped the test mission would redeem the

Starliner program after years of development problems and over

$1.6 billion in budget overruns since 2016.

Boeing ( BA ) is also struggling with quality issues on production

of commercial planes, its most important products.

Veteran NASA astronauts Butch Wilmore and Suni Williams,

both former military test pilots, became the first crew to ride

Starliner on June 5 when they were launched to the ISS for what

was expected to be an eight-day test mission.

But Starliner's propulsion system suffered a series of

glitches beginning in the first 24 hours of its flight to the

ISS, triggering months of cascading delays. Five of its 28

thrusters failed and it sprang several leaks of helium, which is

used to pressurize the thrusters.

In a rare reshuffling of NASA's astronaut operations, the

two astronauts are now expected to return in February 2025 on a

SpaceX Crew Dragon spacecraft due to launch next month as part

of a routine astronaut rotation mission. Two of the Crew

Dragon's four astronaut seats will be kept empty for Wilmore and

Williams.

Starliner will undock from the ISS without a crew and

attempt to return to Earth as it would have with astronauts

aboard.

Boeing ( BA ) struggled for years to develop Starliner, a

gumdrop-shaped capsule designed to compete with Crew Dragon as a

second U.S. option for sending astronaut crews to and from

Earth's orbit.

Starliner failed a 2019 test to launch to the ISS

uncrewed, but mostly succeeded in a 2022 do-over attempt where

it also encountered thruster problems. Its June mission with its

first crew was required before NASA can certify the capsule for

routine flights, but now Starliner's crew certification path has

been upended.

Since Starliner docked to the ISS in June, Boeing ( BA ) has

scrambled to investigate what caused its thruster mishaps and

helium leaks. The company arranged tests and simulations on

Earth to gather data that it has used to try and convince NASA

officials that Starliner is safe to fly the crew back home.

But results from that testing raised more difficult

engineering questions and ultimately failed to quell NASA

officials' concerns about Starliner's ability to make its crewed

return trip - the most daunting and complex part of the test

mission.

NASA's decision, and Starliner's now-uncertain path to

certification, will add to the crises faced by new Boeing ( BA ) CEO

Kelly Ortberg, who started this month with the goal to rebuild

the planemaker's reputation after a door panel dramatically blew

off a 737 MAX passenger jet in midair in January.