WASHINGTON — The launch of Boeing’s CST-100 Starliner spacecraft on its first crewed flight has slipped at least four days because of a helium leak in the spacecraft.

Boeing announced May 14 that the launch of the Crew Flight Test (CFT) mission, previously scheduled for May 17, has been rescheduled for no earlier than May 21 at 4:43 p.m. Eastern on an Atlas 5 from Cape Canaveral Space Force Station in Florida.

The cause of the delay, the company said, was what it described as a “small helium leak” in the spacecraft’s service module, which engineers traced a flange in one of Starliner’s reaction control thrusters. Helium is used to pressurize the propulsion system on the spacecraft.

Boeing did not state in the announcement when the leak was detected, including if it was known during the previous launch attempt May 6 versus some time later, or if the leak would have prevented a launch had it been detected during the countdown. Company spokespersons did not respond to questions about the leak.

The May 6 launch was scrubbed not because of any problems with Starliner but instead with a valve in the rocket’s Centaur upper stage. The rocket was rolled back to the vertical integration facility building near the launch pad, and Boeing said in its statement that the valve was successfully replaced May 11 and tested to confirm it was working properly.

According to the Boeing statement, the company is working with NASA to characterize the leak rather than replace the flange. “As a part of the testing, Boeing will bring the propulsion system up to flight pressurization just as it does prior to launch, and then allow the helium system to vent naturally to validate existing data and strengthen flight rationale,” Boeing stated.

There had been few updates about the status of the mission after the announcement it would be delayed to May 17 to replace the Centaur valve. There had been hints, though, that the mission might slip beyond that date. After previously stating that the CFT crew of Butch Wilmore and Suni Williams would remain at the Kennedy Space Center for the next launch attempt, NASA said May 10 that the two had returned to Houston while staying in a pre-launch medical quarantine. NASA also informed media May 13 that the rollout of the Atlas 5 back to the pad had been delayed a day to May 16. In both cases, though, NASA said the launch remained scheduled for no earlier than May 17.

This incident is the latest in a long string of delays for the program. Various problems with the spacecraft, such as software, valves, parachute components and wiring tape, have all contributed to years of cumulative delays in development of the spacecraft and its test flights. Boeing hopes that, with a successful CFT mission, it will get those problems behind it and enable regular crew rotation missions to the International Space Station as part of a $4.2 billion contract the company received from NASA in 2014.

NASA officials, while eager to start using Starliner and end its reliance solely on SpaceX’s Crew Dragon, emphasized after the May 6 scrub that they were not in a hurry to launch the CFT mission. “We’re not in a rush to fly from a station standpoint. We did clear our summer schedule intentionally to give us plenty of runway for the CFT mission,” Dana Weigel, NASA ISS program manager, said at the briefing after the scrub.

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...