Monday night’s planned launch of the first crewed test flight of the Boeing Starliner spacecraft to the International Space Station was canceled before liftoff because of a technical issue.

Crew members Barry Wilmore and Sunita Williams had just entered the capsule when ground officials scrubbed the launch, noting a faulty oxygen relief valve on the rocket’s second stage.

Wilmore and Williams, veteran astronauts with the U.S. space agency NASA, were to pilot the flight to the space station, where they would stay for about a week to test the capsule and its subsystems.

Officials with United Launch Alliance, the company that built the Atlas V rocket that will carry Starliner into space, said engineers heard an audible buzzing sound from the valve during the countdown that suggested it may have exceeded its usefulness. The company said hours after Monday’s launch was scrubbed that it may be Friday before they will attempt another launch.

The eventual flight will be the final test before NASA can certify Boeing to conduct routine missions to and from the space station for the agency.

If the Boeing test is successful, it will become the second reliable option for human space flight along with Elon Musk’s SpaceX Dragon capsule. SpaceX has been shuttling NASA astronauts to and from the ISS since 2020.

The Starliner program experienced years of setbacks and delays.

During a 2019 uncrewed test flight, the capsule was on an incorrect trajectory and returned without reaching the ISS.

"Two critical software defects were not detected ahead of flight despite multiple safeguards,” NASA said after the failed test flight. “Ground intervention prevented loss of vehicle in both cases."

Another launch was postponed in 2021 because of a valve problem. An uncrewed capsule reached the ISS in May 2022. But the spacecraft has experienced several problems since then, including the discovery of flammable tape in the cabin and weak parachutes.

Some information for this report came from Agence France-Presse.