The crew of Boeing's Starliner spacecraft said Wednesday they're confident the capsule will carry them safely back to Earth at the end of their extended stay aboard the International Space Station, despite helium leaks in the ship's propulsion system and trouble with maneuvering thrusters.

Launched June 5, commander Barry "Butch" Wilmore and co-pilot Sunita Williams originally expected to spend about eight days in space, putting the Starliner through its paces in the ship's first piloted test flight.

But the helium leaks and thruster issues have prompted NASA to extend their stay aboard the space station indefinitely — Wednesday marked their 35th day in orbit  — while engineers carry out tests and analysis to better understand what caused the problems and to make sure the spacecraft can safely being Wilmore and Williams home.

In the meantime, NASA insists the crew isn't "stranded" in space, and both Wilmore and Williams, speaking with reporters for the first time since launch, appeared to agree with that assessment.

"I think where we are right now, and what we know right now, and how the spacecraft flew as it was coming in to do the docking, I feel confident that if we had to, if there was a problem with the International Space Station, we can get in our spacecraft and we can undock, talk to our team, and figure out the best way to come home," Williams said.

"I have a real good feeling in my heart that this spacecraft will bring us home, no problem," she added. "We're learning now to optimize our specific situation and make sure that we know everything about it."

Asked about his faith in the Starliner, Wilmore said "we're absolutely confident."

"We are actually doing thruster testing as we speak at White Sands, New Mexico, trying to replicate (the problems) we saw when we were rendezvousing," he said. "And we are going to learn from that. And we're going to incorporate new processes, new procedures that we will employ if necessary."

But just to be on the safe side, Steve Stich, manager of NASA's Commercial Crew Program, said the team has "dusted off" plans originally developed when the Russian Soyuz spacecraft that carried NASA astronaut Frank Rubio to the station developed a coolant leak.

In that case, preliminary plans were drawn up to possibly bring Rubio down on a SpaceX Crew Dragon capsule if worse came to worse. As it turned out, the Russians launched a replacement Soyuz and those plans were not needed.

"In that case, we decided to use Dragon as a contingency return option for Frank ... to use it as a lifeboat," Stich said. "We had a configuration of Frank in the middeck of Dragon. I mean, certainly we've dusted off a few of those things to look at relative to Starliner, just to be prepared.

"But again, our prime option is to return Butch on Suni on Starliner. ... We're pretty far away from where we were with the Soyuz. We just want to understand the thrusters a little bit more before we commit to the final undock and return."

"Having a great time" in orbit after multiple delays

In the meantime, Williams said she and Wilmore were enjoying the extra time in space.

"We are having a great time here on ISS," she said. "You know, Butch and I have been up here before, and it feels like ... coming back home. It feels good to float around. It feels good to be in space and work up here with the International Space Station team.

"So, yeah, it's great to be up here," she said. "So I'm not complaining, Butch isn't complaining that we're here for a couple extra weeks."

Running four years behind schedule, the Starliner was launched June 5, a month later than planned due to minor problems with the crew's Atlas 5 booster, trouble with a countdown computer and because of a small helium leak in the capsule's aft service module. Helium is used to pressurize the Starliner's propulsion system so jets can fire as needed.

After extensive analysis, NASA and Boeing managers decided the leak was not a safety threat and the Starliner was cleared for launch as is.

Once in orbit and on the way to the space station, however, four more helium leaks developed. In addition, the Starliner's flight computer took several aft-facing maneuvering jets off line when the telemetry did not match pre-set operating parameters.

One thruster was deemed unusable going forward, but the others were successfully test fired later. That "hot-fire" test gave engineers confidence the jets needed for post-undocking maneuvers, and to keep the Starliner steady during the critical de-orbit rocket firing, will work as needed to set the ship up for re-entry.

The thrusters in question were facing the sun during long stretches of the Starliner's approach to the station and engineers suspect the problems experienced earlier were related high temperatures and the rapidity of firings during final approach. They are trying to duplicate those conditions in the ground testing.

As for the helium leaks, the propulsion system isn't used while docked to the space station. The system was pressurized for the hot-fire test, but otherwise valves have been closed as always planned to isolate the helium tanks, eliminating additional leakage.

The system will be repressurized for undocking to enable the maneuvers required to get home. But engineers have said there is 10 times more helium in the tanks than needed for the return trip, providing a comfortable margin even if any of the known leaks got worse.

"I envision that we'll still do testing before we undock, actually first open the helium valves and then secondly once we undock to make sure everything is working correctly, as it's planned from what they found out during the thruster testing," Williams said. "So I have confidence, Butch has confidence."

All of the hardware in question is located in the Starliner's service module, which is jettisoned just prior to atmospheric entry. As such, engineers will never be able to examine the equipment first hand. Williams said it only made sense to extend the mission to do as much testing as possible before coming home.

"If we just came home, we'd lose the SM (service module) and then we wouldn't be able to go through all this testing and understand about our spacecraft," she said.

Stich did not say when the crew might be cleared to undock. The next Crew Dragon crew rotation flight is scheduled for launch in mid August and "a few days before that launch opportunity, we would need to get Butch and Suni home on Starliner."

"So that's kind of a back end," he said. "We're really working to try to follow the data and see when's the earliest that we could we could target for undock and landing. I think some of the data suggests, optimistically maybe, it's by the end of July. But we'll just follow the data (and) figure out when the right undock opportunity is."

In:
• International Space Station
• Elon Musk
• Boeing
• NASA
• SpaceX