As it flew up toward the International Space Station last summer, the Starliner spacecraft lost four thrusters. A NASA astronaut, Butch Wilmore, had to take manual control of the vehicle. But as its thrusters failed, Wilmore lost the ability to move Starliner in the direction he wanted to go.

NASA and Boeing are "working to resolve Starliner’s in-flight anomalies and preparing for propulsion system testing in the months ahead."

Agency describes the flight as a ‘crew-capable post-certification mission’ but acknowledges the possibility of flying without astronauts.