Four astronauts could leave the International Space Station on Sunday before their replacement team arrives to take over, NASA Thursday. However, due to weather conditions, the timing remains uncertain.

The four members of the Crew-2 mission, which include a French and a Japanese astronaut, should return to Earth this month after spending about six months aboard the ISS.

Usually, crew members would have to wait for four other astronauts from the successor mission to replace them, which is the Crew-3. The mission will include three American astronauts and a German.

The takeoff of Crew-3's rocket, which had already been postponed several times and rescheduled for this weekend, was canceled yet again due to unfavorable weather conditions, said a NASA statement.

Due to the repeated postponement, the agency is 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 17:05 GMT, NASA said. 

Without providing a specific time, NASA revealed that a withdrawal opportunity was possible on Monday.

When the capsule the crew is boarding is detached from the ISS, it will begin a journey that lasts for a variable amount of time - but no more than several hours - 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 01:51 am 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 NASA had been quarantining the astronauts 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. 

"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," he added.