Leaving behind the two NASA astronauts it took to the International Space Station three months ago, Boeing’s troubled Starliner spacecraft is set to begin its return to Earth on Friday evening.

Six hours after it undocks from the station, it is to parachute to a landing at the White Sands Space Harbor in New Mexico. If bad weather or technical issues cause a delay, Starliner’s return could happen on Sept. 10, Sept. 14 or Sept. 18.

After lengthy analysis and ground tests, NASA officials said they still did not fully understand the cause of the propulsion system problems that arose as Starliner approached the space station in June.

Both Boeing and NASA officials have said that they expect that the trip back will be uneventful for the empty Starliner. And they maintain that the spacecraft could have probably still brought back safely the two NASA astronauts, Suni Williams and Butch Wilmore, whose stays on the space station have been extended.

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“We have confidence in the vehicle,” Steve Stich, the manager of the commercial crew program at NASA, said during a news conference on Wednesday. He noted that Starliner had successfully landed during earlier test flights with no crew.

“We’ve had two good landings with Starliner so far, and we’re expecting another one Friday,” Mr. Stich said.

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