Veteran astronauts Barry “Butch” Wilmore and Sunita Williams, who were originally assigned to the Starliner Crew Flight Test (CFT), were pulled from the mission by NASA and reassigned to remain on the ISS. Their return to Earth will now be facilitated by a SpaceX capsule in about six months.

Departure from ISS

On September 6, 2023, at 6:04 p.m. EDT, Starliner departed the ISS as Wilmore and Williams watched its final moments at the station. “She’s on her way home,” Williams radioed to ground control as Starliner, named *Calypso*, cleared the station’s safety zone.

The spacecraft’s return to Earth began with the unlatching of 12 hooks that attached it to the Harmony module. Springs in the docking ring helped to push Starliner away as it drifted 260 miles over central China. At a distance of 16 feet, Starliner fired its Reaction Control System (RCS) thrusters, performing a series of 12 burns to separate from the ISS faster than would have been possible with a crew onboard.

Thruster Challenges

This departure marked a contrast from Starliner’s earlier approach to the ISS on June 6, where several RCS thrusters had malfunctioned. However, in the lead-up to the return flight, two additional thruster tests were conducted, and Boeing assured NASA that the issues had been resolved. Despite the improvements, NASA remained cautious, opting to remove the crew from the return mission on August 24.

One critical concern was the potential failure of some of the thrusters, which led to a reassessment of Starliner’s flight computer modeling. NASA’s decision to cancel the crewed flight was based on the uncertainty of the thruster system’s reliability, which will need further analysis before certifying the spacecraft for astronaut missions.

Safe Return to Earth

After circling the planet four times, Starliner initiated its deorbit burn at 11:17 p.m. EDT, slowing the spacecraft’s velocity by 426 feet per second. This maneuver set Starliner on a precise trajectory across the Pacific Ocean, over Mexico’s Baja Peninsula, and eventually to its landing site in New Mexico.

During its descent, Starliner’s service module was successfully discarded for a destructive reentry into the Earth’s atmosphere. A sequence of parachutes was deployed at about 24,500 feet, beginning with two drogue chutes to slow and stabilize the spacecraft, followed by three main parachutes. Airbags inflated at 2,500 feet, cushioning the final touchdown at 12:01 a.m. EDT on September 7.

Addressing Technical Issues

Despite a smooth landing, Starliner’s propulsion system faced challenges throughout the mission. A series of helium leaks were identified, which Boeing traced to degraded seals exposed to corrosive propellants. While the spacecraft had ample helium reserves, further work is necessary to resolve the underlying issues. NASA’s Commercial Crew Program manager, Steve Stich, mentioned that Boeing is exploring different materials and seal sizes to prevent future leaks.

Additionally, the thruster malfunctions that occurred during the spacecraft’s approach to the ISS were linked to overheating in Teflon seals within the thrusters’ valves. The heat buildup was caused by a manual flying test that forced the thrusters to fire in rapid succession while in direct sunlight. Boeing is now working to refine the spacecraft’s flight computer to prevent similar issues in future missions.

Looking Ahead

Despite these setbacks, Starliner’s return to Earth was a significant milestone for Boeing’s crewed spaceflight program. “Many parts of the flight went extremely well, and Starliner is a great spacecraft,” said Stich. “What we really need to do now is examine the areas that didn’t perform as expected and apply those lessons moving forward.”

The spacecraft will undergo servicing at White Sands before being transported back to Boeing’s facility at Kennedy Space Center for further analysis. The timeline for the next crewed mission aboard Starliner remains uncertain, but this uncrewed return has provided crucial data for Boeing and NASA to address before certifying the spacecraft for future astronaut missions.