After a year and a half of focusing on software issues, Boeing has paused its Starliner Orbital Flight Test 2 (OFT-2) mission to the International Space Station today. The launch was scheduled for this afternoon but scrubbed hours before liftoff due to a propulsion system issue with the spacecraft.

Boeing’s mission is to have its uncrewed Starliner spacecraft reach the International Space Station and return safely before it can be rated for human spaceflight. Boeing is one of two providers for NASA’s Commercial Crew Program (along with SpaceX) to transport astronauts to and from the space station.