Tropical weather heading towards Florida could delay this weekend’s planned return of the first SpaceX crew.
On Wednesday, SpaceX and Nasa cleared the Dragon crew capsule to depart the International Space Station and head home after a two-month flight.
NASA test pilots Doug Hurley and Bob Behnken will be aiming for the Atlantic or Gulf of Mexico just off Florida's coast, but the wind and waves must be relatively calm. It would be the first astronaut splashdown in 45 years
However, a hurricane is due to hit Florida at the same time.
The weather system was in the eastern Caribbean on Wednesday and was expected to develop into a tropical storm
Nasa's commercial crew programme manager, Steve Stich, said flight controllers will closely monitor the weather and, if necessary, keep the astronauts at the space station until conditions improve.
“You have to remember this is a test flight,” Nasa Administrator Jim Bridenstine said from Kennedy Space Centre.
"If the weather isn’t good or the sea states aren’t good, we’re going to take our time bringing Bob and Doug home. Our Number One highest priority is their safety.”
Elon Musk's SpaceX company made history on May 30 when launching the pair into space.
It marked the first launch of Nasa astronauts from the US in nearly a decade and also the first time a private company sent people into orbit. SpaceX is already preparing to launch a second crew to the space station at the end of September.
NASA wants six weeks between the splashdown and the launch of the next Dragon crew, for capsule inspections and reviews.