The launch of Boeing’s already delayed Starliner spacecraft is slipping at least four more days, from next Tuesday to May 25 because of ongoing work to resolve concerns about a small helium leak in the capsule’s propulsion system, officials said Friday.

Mission commander Barry “Butch” Wilmore and co-pilot Sunita Williams are now aiming for launch from Pad 41 at Florida’s Cape Canaveral Space Force Station at 3:09 p.m. EDT a week from Saturday, setting up a docking at the International Space Station the next day, May 26, at 4:12 p.m.

The astronauts had hoped to take off on the Starliner’s first piloted test flight on May 6, but the countdown was called off because of trouble with an oxygen pressure relief valve in their Atlas 5 rocket’s Centaur upper stage.

The Atlas 5, built by United Launch Alliance, was hauled from the pad back to the company’s nearby Vertical Integration Facility where the suspect valve was replaced and cleared for launch.

The unrelated helium leak in the Starliner’s propellant pressurization system was noted during the original countdown to launch, but it remained within safe limits for flight. After the Atlas 5 and Starliner were rolled back to the VIF for the oxygen valve replacement, managers decided to take a closer look at the helium issue.

The leak was detected in plumbing making up helium manifold No. 2 inside one of four “doghouse” assemblies spaced around the exterior of the Starliner’s drum-shaped service module. Each doghouse features four Orbital Maneuvering and Attitude Control — OMAC — thrusters, and four small reaction control system maneuvering jets.

Pressurized helium gas is used to push propellants to the rocket motors in each doghouse, as well as to four powerful launch abort engines that would only be fired in the event of a catastrophic problem with the Atlas 5 on the way to orbit.

Engineers tightened bolts in a flange where the leak was detected, pressurized the lines and then ran tests to determine if the leak was still present. In the meantime, launch was re-targeted for May 21.

But as it turned out, tests revealed the leak was still present. Mission managers considered a range of options for resolving the issue, but they decided Friday to press ahead toward a launch opportunity on May 25, pending additional data reviews and analysis to show the leak, which is currently stable and within acceptable limits, will not worsen in flight.

“Pressure testing…showed the leak in the flange is stable and would not pose a risk at that level during the flight,” NASA said in a blog post. “The testing also indicated the rest of the thruster system is sealed effectively across the entire service module.

“Boeing teams are working to develop operational procedures to ensure the system retains sufficient performance capability and appropriate redundancy during the flight. As that work proceeds, (mission managers) will take the next few days to review the data and procedures to make a final determination before proceeding to flight countdown.”

Wilmore and Williams, both veteran Navy test pilots and astronauts with four flights to the station between them, flew back to the Johnson Space Center in Houston after the May 6 scrub to participate in additional flight simulations. They’re expected to return to Florida next week.

The Starliner is one of two commercially developed crew ferry ships ordered by NASA in the wake of the shuttle program’s retirement in 2011. SpaceX won a contract valued at $2.6 billion for development of the company’s Crew Dragon spacecraft and Boeing was awarded $4.2 billion for development of the Starliner.

NASA wanted spacecraft from different builders to ensure the agency would still have a way to get astronauts to the space station even if a problem grounded one company’s ferry ship.

SpaceX launched its first two-man crew in 2020. Since then, the company has launched eight NASA-sponsored crew rotation flights to the station, three commercial research missions to the lab and a privately-funded, two-man, two-woman trip to low-Earth orbit. In all, 50 people have flown to space aboard Crew Dragons.

Wilmore and Williams will be the first astronauts to fly aboard a Starliner after a series of technical glitches that included major software problems during an initial unpiloted test flight in December 2019, and corroded propulsion system valves that delayed a second uncrewed test mission in May 2022.

Engineers ran into questions about parachute harness connectors and protective tape wrapped around wiring that posed a fire risk in a short circuit. Work to correct those issues and others delayed the first piloted launch to this month.