Concerns over a critical hydraulic system arose less than four hours before the Falcon rocket’s planned evening liftoff from NASA’s Kennedy Space Center.

NASA and SpaceX on Wednesday night scrubbed the Falcon 9 launch of a flight intended to retrieve from the International Space Station astronauts Suni Williams and Butch Wilmore, who have spent nine months in orbit.

Tense, public squabbles between SpaceX CEO Elon Musk and NASA have stirred the space industry, but behind the scenes, space science has continued business as (almost) normal.