How Boeing can bring NASA’s Sunita Williams, Barry Wilmore back to Earth

Boeing's Starliner capsule faces propulsion system issues, delaying return of astronauts to Earth from ISS.

Problems with Boeing's Starliner capsule, still docked at the International Space Station (ISS), have upended the original plans for its return of its two astronauts to Earth, as last-minute fixes and tests draw out a mission crucial to the future of Boeing's space division.

NASA has rescheduled the planned return three times and now has no date set for it.

Since its June 5 lift-off, the capsule has had five helium leaks, five manoeuvring thrusters go dead and a propellant valve failed to close completely, prompting the crew in space and mission managers in Houston to spend more time than expected pursuing fixes mid-mission.

Here is an explanation of potential paths forward for Starliner and its veteran NASA astronauts, Barry "Butch" Wilmore and Sunita "Suni" Williams.

Starliner can stay docked at the ISS for up to 45 days, according to comments by NASA's commercial crew manager Steve Stich to reporters. But, if necessary, such as if more problems arise that mission officials cannot fix in time, it could stay docked for up to 72 days, relying on various backup systems, according to a person familiar with flight planning.

Internally at NASA, Starliner's latest targeted return date is July 6, according to a source, who spoke on condition of anonymity. Such a return date would mean that the mission, originally planned for eight days, instead would last a month.

Starliner’s expendable propulsion system is part of the craft’s “service module.” The current problems centre on this system, which is needed to back the capsule away from the ISS and position it to dive through Earth’s atmosphere. Many of Starliner’s thrusters have overheated when fired, and the helium leaks — used to pressurise the thrusters — appear to be connected to how frequently they are used, according to Mr. Stich.