Il Nuovo Postiglione - Earthbound Boeing Starliner faces final challenge in key test mission

The spaceship autonomously undocked at 2:36 pm Eastern time (1836 GMT) and was set to touch down in the New Mexico desert just over four hours later, at 2249 GMT, wrapping up a six-day mission crucial to restoring Boeing's reputation after past failures.

"#Starliner separation confirmed," tweeted Boeing Space.

Orbital Flight Test-2 (OFT-2) is the last hurdle for Starliner to clear before it carries humans in another test flight that could take place by the end of this year.

- Landing airbags -

Starliner docked with the ISS on Friday, a day after blasting off from the Kennedy Space Center in Florida.

Over the weekend, astronauts living aboard the research platform opened the hatch and "greeted" the capsule's passengers: Rosie the Rocketeer, a sensor-equipped mannequin, and a plush toy named Jebediah Kerman, a video game character and the ship's zero-g indicator.

The spacecraft will bring back more than 600 pounds (270 kilograms) of cargo, including reusable tanks that provide breathable air to station crew members, which will be refurbished and taken back up on a future flight.

As it flies over the Pacific Ocean, Starliner will begin a de-orbital maneuver, ejecting its expendable service module which will burn up, while the remaining crew module withstands temperatures of around 3000 degrees Fahrenheit (1650 Celsius) during atmospheric re-entry.

Having shed most of its velocity, it will deploy a sequence of parachutes until, at 3,000 feet, it jettisons its base heat shield -- revealing landing airbags that will inflate to aid a gentle landing.

The mission hasn't been without its hiccups.

Two thrusters responsible for placing Starliner in a stable orbit failed, though officials insisted there was plenty of redundancy built into the system to overcome the problem.

On the day of docking, the vessel missed its scheduled contact time by more than an hour, after a ring responsible for latching on to the station failed to deploy correctly. Engineers had to retract the ring and pop it out again before it worked the second time.

- Second taxi service -

Still, the glitches are minor compared to the troubles Starliner saw during its first test launch, back in 2019, when one software bug caused it to burn too much fuel to reach its destination, and another almost meant that the vehicle was destroyed during re-entry.

The second error was caught in time to upload a patch, and the vessel was able to achieve a gentle landing, slowed by its enormous parachutes, at White Sands Space Harbor -- the same spaceport where the space shuttle Columbia once landed, and where Starliner is once more expected for touchdown.

Boeing and NASA also tried to launch Starliner in August 2021, but the capsule was rolled back from the launchpad to address sticky valves that did not open as they should, and the ship was eventually sent back to the factory for fixes.

NASA is looking to certify Starliner as a second "taxi" service for its astronauts to the space station -- a role that Elon Musk's SpaceX has provided since succeeding in a test mission for its Dragon capsule in 2020.

Both companies were awarded fixed-price contracts -- $4.2 billion to Boeing and $2.6 billion to SpaceX -- in 2014, shortly after the end of the space shuttle program, during a time when the United States was left reliant on Russian Soyuz rockets for rides to the orbital outpost.

T.Cusumano--INP