The launch of Boeing’s star-crossed Starliner spacecraft on its first piloted test flight is slipping to at least June 1 to give engineers more time to assess a small-but-persistent helium leak in the capsule’s propulsion system, and its potential impact across all phases of flight, NASA announced Wednesday.

Already years behind schedule and more than $1 billion over budget, the Starliner’s road to launch has been surprisingly rocky, with multiple problems leading up to its first Crew Flight Test, which is now slipping nearly a month beyond its May 6 target.

That launch attempt was called off because of unrelated trouble with a valve in the United Launch Alliance Atlas 5 rocket that was quickly corrected. But the helium leak in the Starliner’s service module, detected during the May 6 countdown, has proven to be more difficult to resolve to everyone’s satisfaction.

At the time, NASA officials said the leak was within acceptable limits and would not have triggered a launch scrub on its own. But after additional inspections — and an unsuccessful attempt to eliminate the leak by tightening bolts in a flange where it appeared to be originating — mission managers began a more comprehensive analysis.

The helium helps pressurize the spacecraft’s propulsion system, and if too much gas leaks away — the thrusters used for launch aborts, maneuvers in orbit to rendezvous with the International Space Station, and departure and re-entry — all could be affected. While the leak appears to be stable, NASA is looking for “flight rationale” showing it won’t worsen in flight.

“As part of this work, and unrelated to the current leak, which remains stable, teams are in the process of completing a follow-on propulsion system assessment to understand potential helium system impacts on some Starliner return scenarios,” NASA said in a statement late Wednesday.

That will be the subject of a second flight readiness review in the next several days. In the meantime, mission commander Barry “Butch” Wilmore and co-pilot Sunita Williams will remain at the Johnson Space Center practicing procedures in high fidelity flight simulators. They’ll fly back to Florida next week if mission managers clear the Starliner for launch.

A launch on June 1 — the same day SpaceX may be targeting for the next flight of its Super Heavy-Starship rocket — would be set for 12:25 p.m. EDT, roughly the moment Earth’s rotation carries Pad 41 at the Cape Canaveral Space Force Station in Florida into the plane of the space station’s orbit.

If all goes well, the crew would dock at the lab’s forward port on June 2 and return to Earth with a landing at White Sands, New Mexico, on June 10. Assuming no major problems, NASA hopes to certify the Starliner for operational crew rotation flights to the station starting next year, alternating with SpaceX’s Crew Dragon spacecraft.

But that will require a full “human rating certification” for the Starliner, and that will depend on the results of the Crew Flight Test.

“It has been important that we take our time to understand all the complexities of each issue, including the redundant capabilities of the Starliner propulsion system and any implications to our interim human rating certification,” said Steve Stich, manager of NASA’s Commercial Crew Program.

“We will launch Butch and Suni on this test mission after the entire community has reviewed the teams’ progress and flight rationale” at the upcoming flight readiness review, Stich added.