The two astronauts were on hand to help with Starliner’s departure from the space station, which took place as the orbiting lab was flying 260 miles over central China.

“We have your backs, and you’ve got this,” Williams radioed to mission controllers at NASA’s Johnson Space Center in Houston. “Bring her back to Earth. Good luck.”

The return journey was closely watched, as it marked the end of a dramatic few months for Boeing and NASA. The test flight was meant to demonstrate…