A decision to bring Wilmore and Williams home early aboard the Crew 9 ship ahead of Crew 10’s arrival would have left NASA astronaut Don Pettit, who flew to the ISS with a Russian crew last September, as the only American aboard the station, a rare staffing imbalance that NASA has said complicates maintenance of the station’s U.S. components.

NASA and SpaceX on Friday launched a long-awaited crew to the International Space Station that will let them bring home U.S. astronauts Butch Wilmore and Suni Williams, who have been stuck on the orbital lab for nine months.