The Falcon 9 rocket is carrying four astronauts who will replace Wilmore and Williams, both of whom are veteran NASA astronauts and retired U.S. Navy test pilots and were the first to fly Boeing's faulty Starliner capsule to the ISS in June.

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.