Return window closes for Starliner

Two NASA astronauts, Butch Wilmore and Suni Williams, are currently aboard the International Space Station (ISS) following technical issues with Boeing‘s Starliner spacecraft. This complex situation raises concerns and requires a thorough assessment before the astronauts can safely return to Earth.

A helium problem

Starliner’s maiden manned flight took place successfully on June 5 from Cape Canaveral, Florida. Unfortunately, the capsule quickly ran into problems. Engineers identified five leaks helium leaks in the spacecraft’s propulsion system, jeopardizing the safety and stability needed for a safe return. These leaks compromise the propulsion system that is crucial for orbital maneuvers and controlling the spacecraft’s descent to Earth. Helium leak problems in spacecraft, such as the one observed on Starliner, can be attributed to several technical and environmental factors specific to the space environment and the propulsion systems used. Spacecraft are subjected to high variations in temperature, pressure and high radiation levels. These extreme conditions can then exert significant stress on materials and systems, sometimes leading to microcracks or failures in joints and pipes where helium is stored. In the case of Starliner, helium is used as a pressurized gas to power the propulsion systems. It is stored at extremely low (cryogenic) temperatures to maintain its maximum density and energy efficiency. However, cryogenic systems are particularly sensitive to pressure losses and leaks due to the conditions required for storing and handling helium at these temperatures.
Boeing’s CST-100 Starliner spacecraft docked with the International Space Station during the crewed flight test mission. Credit: NASA

Will Starliner make it back?

Boeing and NASA engineers are working to understand the exact causes of these helium leaks and to develop solutions that will ensure the safety of astronauts aboard Starliner. However, the limited fuel in the Harmony module to which Starliner is docked means that it can only remain docked for 45 days. The window for a safe return flight is therefore gradually narrowing. Initially, the capsule’s return to Earth was scheduled for June 13. On June 21, the mission was extended for the first time until June 25. Finally, NASA decided to extend it again until Julywithout giving a specific date. As a reminder, Starliner is part of NASA’s Commercial Crew program, which is intended to restore the United States’ ability to send astronauts to the ISS since the end of the space shuttles in 2011. This program also saw the successful development of SpaceX‘s Crew Dragon, which has completed twelve crewed flights since 2020. For its part, Boeing is not at its first setbacks. A first uncrewed test flight in 2019 was notably affected by a critical software error. The capsule then encountered technical problems with the parachutes and other components.