SpaceX successfully launched four people into space on Friday, beginning a mission that will give the International Space Station enough crew members to allow astronauts Suni Williams and Butch Wilmore to return to Earth after their nine-month stay.

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.