Boeing’s Starliner spacecraft encountered another setback Monday, May 6, as a planned launch was delayed due to a valve issue in the rocket’s upper stage, just hours before liftoff. Commander Barry “Butch” Wilmore and co-pilot Sunita Williams were already preparing for the flight when the scrub was announced.

This test flight — the first manned mission for the Starliner — is intended to ferry astronauts to the International Space Station.

A valve issue in the Atlas 5 rocket’s upper stage postponed the launch. NASA and Boeing are now targeting to launch no earlier than a Friday, May 10. If a valve replacement is needed, the launch may face further delays.

This delay continues a series of challenges for Boeing’s Starliner, which is competing with SpaceX’s operational Crew Dragon spacecraft.

After the launch was canceled, NASA astronauts Wilmore and Williams left the Starliner spacecraft at Cape Canaveral Space Force Station in Florida and returned to the astronaut crew quarters at Kennedy.