NASA officials said on Wednesday the two astronauts delivered to the International Space Station in June by Boeing's Starliner could return on SpaceX's Crew Dragon in February 2025 if Starliner is still deemed unsafe to return to Earth.

The U.S. space agency has been discussing potential plans with SpaceX to leave two seats empty on an upcoming Crew Dragon launch for NASA astronauts Butch Wilmore and Suni Williams, who became the first crew to fly Boeing's Starliner capsule.

The astronauts' test mission, initially expected to last about eight days on the station, has been drawn out by issues on Starliner's propulsion system that have increasingly called into question the spacecraft's ability to safely return them to Earth as planned.

A Boeing spokesperson said that if NASA decides to change Starliner's mission, the company "will take the actions necessary to configure Starliner for an uncrewed return."

Thruster failures during Starliner's initial approach to the International Space Station (ISS) in June and several leaks of helium – used to pressurize those thrusters – have set Boeing off on a testing campaign to understand the cause and propose fixes to NASA, which has the final say. Recent results have unearthed new information, causing greater alarm about a safe return.

The latest test data has stirred disagreements and debate within NASA about whether to accept the risk of a Starliner return to Earth, or make the call to use Crew Dragon instead.

Using a SpaceX craft to return astronauts that Boeing had planned to bring back on Starliner would be a major blow to an aerospace giant that has struggled for years to compete with SpaceX and its more experienced Crew Dragon.

Starliner has been docked to the ISS for 63 of the maximum 90 days it can stay, and it is parked at the same port that Crew Dragon will have to use to deliver the upcoming astronaut crew.

Read more: NASA delays SpaceX Crew-9 mission to address Boeing Starliner issues