Previously planned for late April, the Starliner mission is now slated to launch after a private astronaut mission scheduled for May “as teams assess readiness and complete verification work” for the spacecraft, NASA’s space operations chief Kathy Lueders said on Twitter. She did not provide further details about reasons for the delay.

Starliner’s debut crewed mission, which will carry commander Butch Wilmore and pilot Suni Williams, to the International Space Station will be a crucial moment for Boeing’s space unit. It represents the spacecraft’s final testflight before joining rival SpaceX’s Crew Dragon capsule as the second NASA-approved ride to orbit.

Steve Stich, head of NASA’s Commercial Crew Program, said in an interview with Reuters before the delay was announced that the certification process for the spacecraft had taken “a little longer than we expected” and was “a whole lotta work.”

A successful 10-day test mission with Starliner docked to the space station, an orbital research lab some 250 miles high in Earth’s orbit, would mark a crucial milestone. Boeing has struggled to compete with Elon Musk’s SpaceX in the nascent market for private astronaut flights.

Finding a new launch date after April is complicated by heavy traffic at the space station over the next few months and a tight schedule for Starliner’s launch provider, the Boeing-Lockheed joint venture United Launch Alliance, Boeing and NASA officials have said.

The delay comes as Boeing and NASA performed extra testing on several areas of the spacecraft.

Boeing software engineers are running tests with Starliner’s manual flight system used as a backup in case the spacecraft’s automated flight software fails, Stich said.

A Boeing spokesman said the focus for that testing is for “added redundancy in cases of emergency.”

Deliberations about mission-critical lithium ion batteries and the low chance they overheat while the spacecraft is docked to the station also took more time than expected, Stich said.

In a recent pre-flight technical meeting with Boeing and NASA officials, the space station’s chief safety officer and representatives from NASA’s astronaut office disagreed with Boeing’s plans to proceed with the mission citing concerns over the batteries, according to a person who attended the meetings.

But those NASA officials eventually agreed with Boeing and others at the federal space agency that the chances of a battery mishap that would endanger the crew were low, said the person who requested anonymity to discuss preflight deliberations.

Boeing also is weighing battery redesigns and a plan to add shielding in case one overheats, Stich said. SpaceX, which has already flown seven crewed missions for NASA since 2020, redesigned its spacecraft’s batteries at one point, he said.

“Of course, they have the luxury of having a lot of battery expertise at Tesla,” Stich said, referring to the electric carmaker Musk leads.

Boeing in a statement said on Wednesday it has had no issues with Starliner’s batteries during tests.

“Boeing has conducted more than a dozen Starliner battery thermal runaway tests, stressing the battery cells beyond their intended limit. No issue has surfaced,” the company said.

Stich acknowledged there had been “a little disagreement” during the meetings over how a potential failure of one of the battery’s cells could spread to other cells. He said there have been no test failures, but added sometimes a cell got “a little out of balance” during past tests.

The Starliner battery concerns and expected upgrades, which had not been previously reported, would add to a growing to-do list of tests and redesigns Boeing has faced before it embarks on the long-awaited operational phase of its NASA contract: six astronaut missions over the next few years.

NASA has overseen Starliner’s development under a $4.5 billion contract awarded in 2014. Some 80 software failures cut short an initial, uncrewed Starliner test flight in 2019. The capsule made a successful repeat of that mission in 2022.

Boeing also plans to redesign a system that separates Starliner’s main crew module from its service module, a trunk section containing thrusters that is ditched before the spacecraft returns to Earth, Stich said.

Federal procurement data shows NASA has agreed to pay Boeing at least $24.8 million for the upgrade of that system.

Boeing last year also opted to redesign valves on Starliner’s propulsion system to prevent them from sticking shut prior to launch, which caused a lengthy delay in 2021.

NASA and Boeing’s aim to have the valves redesigned for future missions initiated a dispute with Boeing’s propulsion system supplier. Aerojet Rocketdyne blamed Boeing for the problems, refusing to pay for the redesign, Reuters reported last year.

Boeing has now cut Aerojet from the redesign process and is working directly with Aerojet’s valve supplier, New Jersey-based company Marotta, said a person involved in the process who asked not to be identified.

Aerojet and Marotta declined to comment. Boeing said “we are working with Marotta on a valve redesign.”