Four astronauts could leave the International Space Station on Sunday without their replacement team having arrived to take over, NASA announced Thursday, but the timing remains uncertain due to weather conditions.

The four members of the Crew-2 mission, including a French and a Japanese astronaut, are due to return to Earth this month after spending about six months on board the ISS.

Normally they would have to wait for four other astronauts — three Americans and a German from the Crew-3 mission — to arrive aboard the space station to take their place.

But the takeoff of the next mission’s rocket, which had already been postponed several times and had been rescheduled for this weekend, was once again canceled because of unfavorable weather conditions, NASA said in a statement.

As a result, the space agency is now considering returning Crew-2 to Earth before Crew-3 launches.

“The earliest possible opportunity for undocking” the capsule to bring Crew-2 back to Earth would be at 1:05 pm Sunday Florida time (1705 GMT), NASA said.

A withdrawal opportunity is possible on Monday, NASA added, without giving a specific timetable.

Once detached from the ISS, the capsule will begin a journey of several hours, the duration of which can vary greatly depending on the trajectory, and will then land off the coast of Florida.

The closest launch opportunity for the Crew-3 take off is at 9:51 pm on Monday (0151 GMT Tuesday), but only if NASA does not return Crew-2 on Sunday or Monday.

Crew-3 is scheduled to take off aboard a SpaceX Falcon 9 rocket from Kennedy Space Center in Florida, where astronauts have been in quarantine for days.

“Mission teams will make a final decision on whether to prioritize Crew-3’s launch or Crew-2’s return in the coming days based on the likelihood of favorable conditions,” NASA said.

The two missions are being carried out by NASA in collaboration with SpaceX, which now provides regular missions to the ISS from the United States.

“These are dynamic and complex decisions that change day by day,” said NASA’s Commercial Crew Program manager Steve Stich.

“The weather in November can be especially challenging.”