SpaceX launched the rescue mission on Saturday with a downsized crew of two astronauts and two empty seats reserved for Butch Wilmore and Suni Williams, who will return next year. The Dragon capsule docked in darkness as the two craft soared 265 miles (426 kilometers) above Botswana.

Because NASA rotates space station crews approximately every six months, this newly launched flight with two empty seats reserved for Wilmore and Williams won’t return until late February. Officials said there wasn’t a way to bring them back earlier on SpaceX without interrupting other scheduled missions.

A SpaceX Crew Dragon space capsule, which is due to bring home stuck astronauts Butch Wilmore and Suni Williams next year, arrived at the International Space Station (ISS) on Sunday, according to NASA and SpaceX.

Two astronauts — one American, one Russian — are headed to the International Space Station with two empty seats for crewmates who are already in orbit.

Two astronauts relinquished their seats on a four-person spacecraft so that their colleagues could return to Earth from the ISS, where they’ve been stuck since June.