The Boeing Starliner crew capsule is due to return to Earth on Sunday as its first test flight is cut short by an improperly set clock.
The capsule – carrying no astronauts and only a test dummy – was supposed to spend the coming week at the International Space Station as part of a critical dress rehearsal for a flight with a real crew next year.
But it ended up in the wrong orbit shortly after launching on Friday. The station docking was scrapped, and Boeing and Nasa decided to bring the spacecraft home as soon as possible.
Flight controllers are working towards a pre-dawn touchdown in the New Mexico desert – always the planned landing site but six days earlier than expected.
After about 50 hours in space, the Starliner will fly over Mexico’s Baja Peninsula, across northern Mexico and then west of El Paso, Texas, into New Mexico for its touchdown.
The orbital demonstration was supposed to be Boeing’s chance to catch up with SpaceX, which completed a similar flight test last March.
Nasa hired the two companies five years ago to transport its astronauts to and from the space station in the wake of the end of the shuttle programme, but commercial crew flights have been hit by numerous technical problems and delays.
Jim Chilton, a senior vice-president at Boeing, said on Saturday that the timing problem in the Starliner’s automated system has since been corrected and the re-entry and landing systems have checked out well.
For unknown reasons, the Starliner’s clock did not sync up properly with the timing on the Atlas V rocket that launched the capsule.
“We started the clock at the wrong time,” Mr Chilton explained on Saturday. “As a result of starting the clock at the wrong time, the spacecraft upon reaching space thought she was later in the mission and, being autonomous, started to behave that way.”
The Starliner burned excess fuel trying to orient itself in orbit. Flight controllers tried sending commands to fix the problem, but the capsule was in a bad position to receive them and there was also a gap in communication coverage.
By then it was too late, and there was not enough on-board fuel to permit a space station visit.
During a launch abort test landing last month in New Mexico, only two of Starliner’s three main parachutes deployed because of a rigging mistake.
Mr Chilton said workers verified the rigging for this Starliner, and Nasa checked photos to make certain.
Nasa administrator Jim Bridenstine said the upcoming landing is a major objective for Starliner, and all staff involved need to be on their “A game”.
Nasa has yet to decide whether Boeing will need to conduct another orbital test flight – and actually dock with the space station – before putting crews on board.
Before the current flight, the first astronaut flight had been pencilled in for next summer.
SpaceX, meanwhile, is planning its first launch with Nasa astronauts by spring.
In a tweet to Boeing, SpaceX founder and chief executive Elon Musk said: “Orbit is hard. Best wishes for landing & swift recovery to next mission.”