WASHINGTON, July 10 (Reuters) - The first two astronauts

to fly Boeing's ( BA ) Starliner capsule said from the

International Space Station on Wednesday they were confident in

the spacecraft's ability to return them home whenever the

company and NASA fix an array of thruster issues that have kept

them in space far longer than expected.

"I have a real good feeling in my heart that this spacecraft

will bring us home, no problem," NASA astronaut Sunita "Suni"

Williams said during the test crew's first news conference since

docking to the ISS more than a month ago.

Williams and Barry "Butch" Wilmore, both veteran NASA

astronauts and former U.S. Navy test pilots, were launched

aboard Starliner from Florida on June 5 and docked the next day

at the ISS, where they were initially scheduled to spend roughly

eight days.

A series of issues with Starliner's propulsion system has

extended their mission indefinitely. Five of Starliner's 28

maneuvering thrusters went dead during its 24-hour trek to the

station, a propellant valve failed to properly close and there

have been five leaks of helium, which is used to pressurize the

thrusters.

"We're absolutely confident," Wilmore told reporters. "That

mantra you've heard, failure is not an option."

"And that's why we're staying, because we're going to test

it. That's what we do," Wilmore said, acknowledging that an

ongoing investigation by the U.S. space agency and Boeing ( BA )

involving thruster tests on Earth is key for their return.

The current test mission is Boeing's ( BA ) final step

before the spacecraft can clinch NASA certification for routine

astronaut flights and become the second U.S. orbital capsule

alongside SpaceX's Crew Dragon, which has dominated the nascent

human spaceflight market amid Starliner's development delays.

NASA officials and Boeing ( BA ) engineers have focused on the

faulty thrusters and aim to conduct weeks of further testing

before allowing Starliner to return Wilmore and Williams to

Earth. That testing includes firing identical thrusters at the

White Sands Missile Range in New Mexico to get insight into what

might be plaguing the thrusters in space.

"Once that testing is done, then we'll look at the plan for

landing," NASA's commercial crew chief Steve Stich told

reporters last month. "We're not going to target a specific date

until we get that testing completed."

The testing could last "a couple weeks" or more, followed by

a NASA review of the resulting data, Stich said. Starliner is

approved to stay docked to the ISS for 45 days, or up to 90 days

using various backup systems and depending on the health of its

lithium ion batteries, which have caused concerns in the past.

Though NASA and Boeing ( BA ) have said Starliner is capable of

returning the astronauts to Earth in the event of an emergency

on the ISS, the capsule is not approved to fly home under

normal, non-emergency circumstances until its thruster issues

are resolved or at least better understood.

NASA and Boeing ( BA ) officials have emphasized that the two

astronauts are not stranded in space.

A Russian satellite last month broke apart into some 180

pieces of debris near the space station's orbit and forced

astronauts into their various docked spacecraft, including

Wilmore and Williams getting into Starliner, to prepare for a

potential escape. Boeing ( BA ) cited the event as an example of

Starliner's readiness to return home if absolutely necessary.

"Starliner stood ready to undock and return Wilmore and

Williams to Earth if needed," the company said in a statement

last month.

The debris risks waned and astronauts emerged from their

capsules an hour later.