Four astronauts from US, France and Japan become first sent into orbit by a rocket booster used in previous spaceflight.
Four astronauts have left the International Space Station on board a SpaceX vessel, after more than 160 days in space which will culminate in a splash landing off the Florida coast.
The Crew Dragon capsule undocked from the ISS as scheduled at 8:35pm on Saturday (00:35 Sunday GMT).
With the flight back to Earth expected to take six and a half hours, the crew was scheduled to splash down in the dark of night early on Sunday in the Gulf of Mexico, just off Panama City, Florida.
“Dragon separation visually confirmed,” a NASA commentator said after two sets of six hooks tying the capsule to the ISS retracted.
The capsule then fired a series of short bursts with its thrusters to gently ease away from the ISS.
NASA livestream footage showed the Crew Dragon capsule moving off into the dark as it began its journey back to Earth, its rear engines lighting up in small flashes.
Seven astronauts remained on the ISS, including a new crew of four who arrived on a different SpaceX craft last week.
“Thanks for your hospitality,” Michael Hopkins, one of the departing US astronauts, said as the capsule moved away. “We’ll see you back on Earth.”
NASA and SpaceX have alternative splash down sites ready, other than Panama City, if need be.
“We have been practising to recover the crews day or night,” Steve Stich, NASA’s Commercial Crew Program manager, said shortly before the capsule’s departure.
“The ships have lots of lighting,” helped by “good moonlight”, he said, adding that weather conditions were excellent, with calm seas.
SpaceX boats are expected to reach the capsule about 10 minutes after splashdown.
Astronauts Hopkins, Victor Glover, Shannon Walker and Japan’s Soichi Noguchi went to space last November as the crew on the first fully operational mission to the ISS on board a vehicle made by Elon Musk’s SpaceX, which has become NASA’s favoured commercial transportation partner.
Prior to that, two American astronauts made a test mission to the ISS in May and stayed for two months.
That was the first launch to the ISS from US soil since the end of the Space Shuttle programme in 2011. It was also the first crewed mission run by a private company, as opposed to NASA.
Until now, US astronauts had been catching rides to the ISS on board Russian spacecraft since the Space Shuttle programme ended.