Follow CNN’s live updates on Boeing Starliner capsule returns home.

CNN
—

Boeing’s Starliner capsule undocked from the International Space Station on Friday evening, ending its nearly three-month stay in space. But it is flying with an empty cabin and leaving behind two test pilots who must now stay on the station for another five or six months.

The Starliner left its docking site at the space station shortly after 6 p.m. ET, according to NASA. The capsule will fly freely through orbit for about six hours as it slowly begins its descent toward home.

Before the capsule’s departure, Butch Wilmore and Suni Williams – the two NASA astronauts who flew to the space station on the Starliner in June – wished the spacecraft, which Williams named “Calypso,” good luck on its long-awaited and unexpected journey home.

“It’s time to bring Calypso home,” Williams told mission control Friday night. “You can do this. We’re behind you and you can do this. Bring her back to Earth.”

Around midnight, the capsule is expected to reach one of the most important and dangerous stages of its test flight: reentry. This milestone requires the Starliner to carefully orient itself as it plunges into the densest part of Earth’s atmosphere while still traveling at orbital speeds – typically more than 17,000 miles per hour.

As with any spacecraft returning from orbit, the pressure and friction of reentry place enormous stress on the vehicle. The process can heat the outside of the spacecraft to over 3,000 degrees Fahrenheit (1,649 degrees Celsius).

As the Starliner then free-falls through the air, a set of parachutes – which Boeing has newly designed and tested in January – must safely slow the capsule before the vehicle deploys its airbags and gently lands on solid ground.

Starliner is designed to be the first U.S.-made capsule to land on Earth by parachute rather than splashing down at sea. Boeing hopes this approach will make it easier to recover and overhaul the spacecraft after flight.

The planned landing site for the vehicle is in New Mexico at White Sands Space Harbor, an area within a massive rocket base of the same name that was previously used to train NASA space shuttle pilots. According to the space agency’s latest schedule, Starliner is expected to reach its destination around 12 p.m. ET.

Concerns about the Starliner’s ability to maintain precise orientation upon re-entry into Earth’s atmosphere — given several problems encountered during the first leg of its journey — are one reason NASA opted not to fly the Starliner crew home on the vehicle. Instead, the new plan calls for Williams and Wilmore to remain aboard the space station and fly home on a SpaceX capsule in February 2025.

NASA officials initially expressed confidence that they could pinpoint the Starliner’s problems and develop a plan for the vehicle’s safe return with its crew. But two lingering problems ultimately derailed those plans.

First, a series of helium leaks occurred during the Starliner’s outbound flight in early June. Helium is used to pressurize some of the vehicle’s thrusters, which help the spacecraft maintain its orientation in space, and problems with leaking helium delayed that crewed test mission even before launch.

In addition, five of the 28 engines of the Starliner’s “Reaction Control System” (RCS) suddenly failed en route to the space station. Four were eventually recovered, but at least one will remain out of service for the entire mission.

NASA and Boeing kept the Starliner in space for weeks to understand these issues, extending William and Wilmore’s stay from the expected eight days to months.

At one point, NASA and Boeing said they believed they understood the likely cause of the Starliner’s problems. The “doghouses,” the structures that contain propulsion equipment, were getting hotter than expected, causing some Teflon seals in the engine systems to bulge — restricting fuel flow and causing RCS engine failures. Regardless, officials said, the helium leaks may have been caused by seals that were damaged by contact with fuel vapors.

However, determining the root cause of a spacecraft problem is not an exact science.

And even if the problems were well understood, the Starliner team had to face the fact that they would never have the opportunity to inspect the actual vehicle’s propulsion system in space. It was impossible to investigate the problem while the Starliner was attached to the space station.

In addition, the unit that houses the problematic RCS engines and the helium leaks – the so-called service module – was not designed to survive the flight home. Instead, the service module, a cylindrical attachment on the floor of the crew cabin, is designed to be jettisoned and disposed of on the way back to Earth. And that’s exactly what it will do on this mission.

The uncertainty about what exactly happened to the service module components was a key factor in NASA’s decision to fly the vehicle home without a crew.

“The bottom line on whether to bring Starliner back is simply that there were just too many uncertainties in predicting the engines,” Steve Stich, head of NASA’s Commercial Crew Program, said Aug. 24. “The risk with the crew was just too great, and that’s why we decided to continue down the unmanned path.”

During its perilous return to Earth, Starliner will have other types of thrusters designed to help the vehicle maintain its orientation during its journey through space.

In addition to the 28 RCS engines, there are 20 Orbital Maneuvering and Attitude Control (OMAC) engines, each of which has about 17 times the thrust of an RCS engine.

But during an earlier press conference, Stich described how the combination of problems already identified with the Starliner’s service module could lead to a catastrophic scenario.

“The worst case scenario would be an integrated failure mechanism between the helium leaks and the RCS thrusters,” Stich said. “For a nominal deorbit burn, we fire 10 OMAC thrusters in each of the four doghouses, and then the RCS jets are just there to maintain alignment.”

However, Stich added, the outcome could be dire if OMAC’s engines fail due to helium leaks.

“Then it could be that there are some cases that are not so easy to bring under control, and those are the really more stressful cases that are causing concern for the team,” Stich said.

“Stay alert”

During the August 24 press conference, NASA officials also indicated that Boeing disagreed with some of the space agency’s risk assessments.

There was “just a little bit of a disagreement (between NASA and Boeing) about the level of risk,” Stich said. “It just depends on how you assess the risk. … We handled it a little differently with our crew than Boeing did.”

But Ken Bowersox, deputy director of NASA’s Space Mission Directorate, added that the company had left the final decision to NASA “based on our broader view of all the risks involved.”

Boeing said as recently as August 2 that it “remains very confident” that Starliner can return with a crew. But since NASA announced its decision to fly the capsule home without astronauts, the company has only said it is focused on “executing the mission as defined by NASA and preparing the spacecraft for a safe and successful uncrewed return,” according to an August 24 statement.

Although the Starliner’s flight home without a crew eliminates a certain risk of disaster, “we are still in the middle of a test flight. We have to remain vigilant,” Bowersox said.

How Starliner performs on its journey home could have far-reaching consequences for the future of the Boeing program. The company has already lost more than a billion dollars fixing problems with the aircraft due to years of delays, development setbacks and flight errors.

It is not yet clear whether NASA will require Boeing to repeat this test flight – or whether the space agency will consider approving the spacecraft for regular flight if the return flight tonight goes smoothly.

Boeing has already had to repeat one test mission, an unmanned test flight that launched in 2019. Software problems prevented the vehicle from entering the correct orbit and even reaching the International Space Station. NASA therefore asked Boeing to repeat the flight in 2022.

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advances and more.