Follow CNN's live updates on the Boeing Starliner capsule returns home.
Boeing's Starliner capsule undocked from the International Space Station Friday night, ending its nearly three-month stay in space. But it's flying with an empty cabin, leaving two test pilots who now must remain on the station for another five or six months.
According to NASA, the Starliner left its docking port at the space station just after 6 p.m. ET. The capsule will fly in free orbit through space for about six hours as it slowly descends toward home.
Before the capsule took off, Butch Wilmore and Suni Williams - the two NASA astronauts who flew to the space station aboard Starliner in June - wished the spacecraft, nicknamed "Calypso" by Williams, 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."
By midnight, the capsule is expected to reach one of the most important and dangerous stages of its test flight: atmospheric reentry. This milestone will require the Starliner to carefully orient itself as it plunges into the thickest part of Earth's atmosphere while still traveling at orbital speeds - typically more than 17,000 miles per hour (27,400 kilometers per hour).
As with any spacecraft returning from orbit, the pressure and friction of reentry into the atmosphere will place immense stress on the vehicle. The process can heat the outside of the spacecraft to over 3,000 degrees Fahrenheit (1,649 degrees Celsius).
Then, as the Starliner falls through the air, a set of parachutes - which Boeing redesigned and tested as recently as January - will safely slow the capsule before the vehicle deploys its airbags for a soft landing on solid ground.
Starliner is the first U.S.-made capsule to land on the ground with a parachute rather than splashing down in the ocean. Boeing hopes this approach will make it easier to recover and refurbish the spacecraft after its flight.
The vehicle's intended landing site is in New Mexico at White Sands Space Harbor, an area within a large rocket range of the same name that was previously used to train NASA space shuttle pilots. Starliner is expected to reach its target around 12 a.m. ET, according to the space agency's latest timeline.
The Troubled Starliner Test Flight
Concerns about Starliner's ability to maintain precise orientation during re-entry into Earth's atmosphere - given the various problems encountered during the first leg of the journey - are among the reasons NASA opted not to fly the Starliner crew home in the vehicle. Instead, the new plan is for Williams and Wilmore to remain aboard the space station and hitch a ride home in a SpaceX capsule in February 2025.
Initially, NASA officials expressed confidence that they could identify Starliner's problems and devise a plan to safely return the vehicle with a crew. But two persistent problems ultimately threw a spanner in the works.
First, a series of helium leaks occurred during Starliner's outbound flight in early June. Helium is used to pressurize some of the vehicle's thrusters, which help the spacecraft stay oriented in space, and problems with some of the helium leaking out delayed this crewed test mission even before liftoff.
Separately, five of Starliner's 28 "reaction control system" (RCS) thrusters abruptly stopped working 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 gain insight into the problems, which meant that Williams and Wilmore's stay did not last longer than expected, namely eight days, but ultimately months.
At one point, NASA and Boeing said they believed they understood the likely cause of the Starliner's problems. The "doghouses," or structures that house propulsion equipment, were flying hotter than expected, causing some Teflon seals in the thrust systems to bulge, restricting the flow of propellant and causing the RCS propellant to fail. Separately, officials said the helium leaks may have been caused by seals that had deteriorated from exposure to propellant vapor.
However, determining the 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 be able to inspect the propulsion system of the actual vehicle in space. It was impossible to investigate the problem while the Starliner was attached to the space station.
Furthermore, the unit housing the troubled RCS thrusters and the helium leaks - called the service module - was not intended to survive the trip home. Instead, the service module, a cylindrical fixture at the bottom of the crew cabin, is designed to be ejected and discarded on the way back to Earth. And that is what it will do on this mission.
The risks that lie ahead
The uncertainty over what exactly happened to the service module's components was a key factor in NASA's decision to fly the vehicle home without a crew.
"The crux of the matter of bringing Starliner back is that there was just too much uncertainty in the prediction of the thrusters," Steve Stich, manager of NASA's Commercial Crew Program, said Aug. 24. "It was just too risky with the crew, and so we decided to go the unmanned path forward."
During its perilous return to Earth, Starliner carries other types of thrusters that help the spacecraft maintain its orientation as it travels through space.
In addition to the 28 RCS thrusters are 20 "Orbital Maneuvering and Attitude Control" or OMAC thrusters, each of which has about 17 times the thrust of an RCS thruster.
But during an earlier press conference, Stich described how the problems already identified with the Starliner's service module could combine to create a disaster scenario.
"The worst case would be an integrated failure mechanism between the helium leaks and the RCS thrusters," Stich said. "For a nominal de-orbit burn, we're firing 10 OMAC thrusters in each of the four doghouses, and then the RCS jets are just there to hold orientation."
But it could be a bleak outcome, Stich said, if the OMAC thrusters were to fail due to helium leaks.
"Then you can end up with cases that are not easy to control, and those are the most stressful cases that the team worries about," Stich said.
'Stay vigilant'
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 evaluate the risk. ... We did it a little bit differently with our crew than Boeing did."
But Ken Bowersox, NASA's deputy director for the Space Mission Directorate, added that the company left the final decision to NASA "because of our broader view of all the risks involved."
Boeing said on August 2 that it "remains confident" that Starliner can return with a crew. But since NASA announced its decision to fly the capsule home without astronauts, the company has said only that it is focused on "executing the mission as defined by NASA, and we are preparing the spacecraft for a safe and successful uncrewed return," according to an August 24 statement.
While flying Starliner home without a crew eliminates some of the risk of catastrophe, "we're still in the middle of a test flight. We have to remain vigilant," Bowersox said.
How Starliner performs on its return trip could have broader implications for the future of Boeing's program. The company has already lost more than $1 billion fixing problems with the vehicle, amid years of delays, development setbacks and in-flight failures.
It's not yet clear whether NASA will require Boeing to repeat this test flight, or whether the space agency will consider certifying the spacecraft for a regular flight if the return trip goes well tonight.
Boeing has already had to repeat a test mission, an unmanned test flight that began in 2019. Software problems prevented the spacecraft from entering orbit or even reaching the International Space Station, prompting NASA to demand that Boeing repeat the excursion in 2022.
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