(CNN) – Boeing's Starliner capsule undocked from the International Space Station on Friday night, concluding its nearly three-month stay in space. But it flew off with an empty cabin, leaving behind two astronauts who must now remain on the station for another five or six months.
The Starliner left its docking port on the space station shortly after 6 p.m. Miami time, according to NASA. The capsule spent about six hours flying in free orbit as it slowly descended toward home.
“It’s time to bring Calypso home,” Williams told mission control Friday night. “You can do it. We’ll have your back, and you’ll get it done. Bring her back to Earth.”
Near midnight, the capsule reached one of the most important and treacherous stages of its test flight: reentry. The milestone required Starliner to carefully orient itself as it plunged 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 all spacecraft returning from orbit, the pressure and friction of reentry put enormous strain on the vehicle. The process could have heated the exterior of the craft to more than 3,000 degrees Fahrenheit (1,649 degrees Celsius).
Before the capsule departed, Butch Wilmore and Suni Williams — the two NASA astronauts who flew Starliner to the space station — wished the craft, nicknamed “Calypso” by Williams, luck on its journey home in June.
The vehicle landed in New Mexico at the White Sands Spaceport, an area within a vast missile range of the same name that was once used to train NASA space shuttle pilots. The Starliner touched down at 12:01 a.m. ET Saturday, according to the space agency.
“I want to acknowledge the work the Starliner teams did to ensure a successful and safe undocking, orbit, reentry and landing,” said Mark Nappi, vice president and program manager for Boeing’s Commercial Crew Program. “We will review the data and determine the next steps for the program.”
During a press briefing after the landing, NASA Commercial Crew Program Manager Steve Stich said it’s not clear exactly what the path forward is for Starliner — or when the spacecraft might be able to fly again. That “will take a little bit of time,” he said.
However, he added: “We have now landed a capsule on solid ground in the United States. The entry, in particular, has been almost perfect, which has not surprised me, but when I think about the mission, the entry itself has been perfect, and the deorbit burn has been perfect.”
Stich also acknowledged that if the ground modeling had been perfect, perhaps NASA would have relied on Starliner to bring Wilmore and Williams home on the vehicle's return flight.
“If we had a model that perfectly predicted what we saw tonight, yes, it seems like an easy decision to say we could have had a manned flight, but we didn’t,” Stitch said.
“From a human perspective, we were all happy that the landing was successful,” he said. “But there is a part of us, all of us, that wishes it had gone the way we had planned. We had planned for the mission to land with Butch and Suni on board.”
Listen to the “strange noise” coming from the Boeing Starliner that prompted astronauts to alert mission control
Concerns about Starliner’s ability to maintain a precise orientation as it reenters Earth’s atmosphere, given several issues that arose during the first leg of its journey, is one reason NASA opted not to fly the Starliner crew back with the vehicle. Instead, the new plan is for Williams and Wilmore to remain aboard the space station and return home in a SpaceX capsule in February 2025.
At first, NASA officials expressed confidence that they could identify Starliner's problems and come up with a plan to safely return the vehicle with the crew. But two persistent issues ultimately derailed those plans.
First, a series of helium leaks emerged during Starliner’s outbound flight in early June. Helium is used to pressurize some of the vehicle’s propellants, which help the craft stay oriented in space, and problems with some of the helium leaks delayed this crewed test mission even before liftoff.
Additionally, five of Starliner’s 28 “reaction control system” (RCS) thrusters abruptly stopped working en route to the space station. Four were recovered, but at least one will remain out of service for the duration of the mission.
NASA and Boeing kept Starliner in space for weeks as they worked to understand those 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 Starliner’s problems. The “decks,” or structures that hold the propulsion equipment, were flying hotter than expected, causing some Teflon seals inside the thruster systems to bulge, restricting propellant flow and causing the RCS thruster failures. Separately, officials said the helium leaks could have been caused by seals that degraded from exposure to propellant vapor.
Determining the root cause of a problem on a spacecraft, however, is not an exact science.
And even if the problems were well understood, the Starliner team had to face the fact that it would never be able to inspect the propulsion system on the actual vehicle in space. It was impossible to examine the problem while Starliner was docked to the space station.
Also, the unit that houses the problematic RCS thrusters and leaking helium, called the service module, was not intended to survive the return trip. Instead, the service module, which is a cylindrical fixture on the bottom of the crew cabin, is designed to be ejected and discarded on the way back to Earth. And that's what it will do on this mission.
Ambiguity over what exactly happened to the service module components was a key factor in NASA's decision to fly the vehicle back without its crew.
“The bottom line regarding bringing Starliner back is that there was too much uncertainty in the thruster prediction,” Steve Stich, NASA’s Commercial Crew Program manager, said Aug. 24. “It was too much risk with the crew, so we decided to go the uncrewed route.”
During its perilous return to Earth, Starliner has other types of thrusters intended to help the vehicle maintain its orientation as it travels through space.
Along with the 28 RCS thrusters are 20 Orbital Maneuvering and Attitude Control (OMAC) thrusters, each of which has approximately 17 times the thrust of an RCS thruster.
But during an earlier press conference, Stich described how problems already identified with the Starliner service module could combine to create a disastrous scenario.
“The worst case would be some built-in 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 deckhouses, and then the RCS jets are there just to maintain orientation.”
During Starliner's final descent on Friday, a series of checks conducted by ground teams showed that one of the thrusters on board the Starliner capsule itself had run out of power.
The previous thruster failures that concerned Boeing and NASA teams occurred in the Starliner service module, a cylindrical attachment at the base of the capsule.
During an Aug. 24 press briefing, NASA officials also indicated that Boeing disagreed with some of the space agency's risk assessments.
There was “just a little bit of disagreement (between NASA and Boeing) in terms of the level of risk,” Stich said. “It depends on how you assess the risk. (…) We did it a little bit differently with our crew than Boeing did.”
But Ken Bowersox, NASA associate administrator for the space operations mission directorate, added that the company left the final decision to NASA “because of our broader view of all the risks involved.”
Boeing said as recently as Aug. 2 that its “confidence remains high” that Starliner can return with a crew. But since NASA announced its decision to bring the capsule back without astronauts, the company has said only that it is focused on “executing the mission as determined by NASA, and we are preparing the spacecraft for a safe and successful uncrewed return,” according to an Aug. 24 statement.
The Starliner's performance 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, over years of delays, development setbacks and in-flight errors.
It is not yet clear whether NASA will require Boeing to repeat the test flight, or whether the space agency will consider certifying the craft for regular flight after the mission lands safely.
Boeing has already had to repeat one test mission, an uncrewed test flight that took off in 2019. Software issues prevented the vehicle from entering the correct orbit and even reaching the International Space Station, prompting NASA to require Boeing to repeat the excursion in 2022.
