The SpaceX commercial astronaut capsule has undocked from the International Space Station (ISS) and will splash down in the Atlantic Ocean.
The demonstration flight, whose lone occupant is a test dummy named Ripley, is a crucial step in Nasa’s delayed quest to resume human space flight from the US this year.
The Crew Dragon capsule has been docked to the ISS for the past week and is heading back towards Earth.
It is equipped with a heat shield to protect it from the high temperature re-entry and four parachutes to bring it into soft contact with the ocean about 280 miles (450km) from Cape Canaveral in Florida.
Splashdown is expected at about 1345 GMT (0845 EST) on Friday. A boat, the GO Searcher, will be waiting to recover the capsule.
The mission has so far gone smoothly. After docking to the ISS, the Canadian astronaut David Saint-Jacques was the first to enter the capsule, describing it as a “business-class” experience.
The capsule’s parachute system and heat shield are relatively untested. And there is a chance that the shield’s irregular shape could cause instabilities at hypersonic speeds.
“I see hypersonic re-entry as probably my greatest concern,” Musk said on Saturday, after the Crew Dragon was launched by a SpaceX Falcon 9 rocket.
Since the retirement of Nasa’s space shuttle programme eight years ago, American astronauts have relied on Russian rockets to travel to the ISS. Nasa hopes to use commercial SpaceX rockets from this year.
Ripley is covered in sensors to monitor the forces that human astronauts would be subject to on a similar flight.
The demonstration flight follows news that Musk’s security clearance is being reviewed by the Pentagon after he smoked marijuana on a Californian comedian’s podcast in September.
An incident report was started by the Pentagon some time after the incident, according to an American official, who spoke on condition of anonymity.