WASHINGTON — The first flight of Boeing’s CST-100 Starliner commercial crew vehicle with astronauts on board, previously scheduled for late April, has been delayed, likely until the summer.

In a tweet March 23, Kathy Lueders, NASA associate administrator for space operations, said that Starliner’s Crew Flight Test (CFT) mission had been delayed to some time after the launch of Axiom Space’s Ax-2 private astronaut mission to the International Space Station in early May.

Lueders said the delay in the launch of CFT, which had been planned for the later half of April, would allow teams to “assess readiness and complete verification work” but did not offer additional details about the delay, or why it was announced now. She said further details would come after the revised schedule was finalized.

It was not clear what triggered the announcement, with no details provided by NASA or Boeing. Industry sources said that pre-flight checkouts and other work, such as software testing, had taken longer than planned, but were unaware of any specific development that prompted the NASA announcement.

At a Feb. 17 briefing, NASA and Boeing officials said work was on schedule for a launch in mid to late April. At the time, the next major milestone was fueling of the spacecraft, which officials said they wanted to do within 60 days of the scheduled launch to mitigate any leaks that could corrode valves, a problem that delayed the vehicle’s second uncrewed test flight by more than 10 months. That fueling was scheduled to take place in early March.

However, neither the company nor the agency had announced that fueling had taken place, and agency officials hedged about the schedule when asked about progress on the mission. “We are in the middle of final preps for the CFT flight, in the middle of closing out all of the certification work,” Steve Stich, NASA commercial crew program manager, said in a March 11 briefing after the splashdown of the SpaceX Crew-5 mission. That included the “final phases” of flight software testing, he said.

He noted that he and others in the commercial crew program had been focused on the launch of the SpaceX Crew-6 mission and return of Crew-5, and that he was not up-to-date on the progress Boeing was making on CFT. “We really need to step back here in March and take a look at where we’re at and determine what the next steps are,” he said. “Right now we’re targeting a no-earlier-than launch date of the end of April.”

Phil Dempsey, NASA ISS transportation integration manager, offered a similar schedule at a March 13 briefing ahead of a SpaceX cargo Dragon launch to the station. NASA and Boeing “were talking over the readiness of the vehicle,” he said. “There is a handful of open work that is still being assessed. The current timing is no earlier than the end of April, April 30 right now, but that is under review at the moment based on the readiness of the vehicle.”

NASA and Boeing had not provided updates on the status of the CFT mission since those briefings. There had been no other updates regarding preparations for the mission.

That included the status of the Atlas 5 launch vehicle that will launch Starliner. A launch in late April would have put it in conflict with the inaugural launch of United Launch Alliance’s Vulcan Centaur rocket, currently scheduled for as soon as May 4. Vulcan and Atlas use the same launch pad at Cape Canaveral Space Force Station, and ULA has been conducting tests of the Vulcan rocket on that pad. It has not shared updates on the status of the Atlas 5 used for Starliner.

During a panel at the Satellite 2023 conference March 15, Tory Bruno, chief executive of ULA, said he was still confident that Vulcan would be ready for launch in May, based on tests on the pad and qualification tests of the vehicle’s BE-4 engines.

CFT will fly NASA astronauts Butch Wilmore and Suni Williams to the station for a mission scheduled to last eight days. It is the final flight test before NASA certifies the vehicle for use in ISS crew rotation missions starting no sooner than early 2024.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...