NASA is still deciding whether to keep two astronauts at the International Space Station until early next year and send their troubled Boeing capsule back empty.
Rather than flying Boeing's Starliner back to Earth, Butch Wilmore and Suni Williams would catch a ride on SpaceX's next flight. That option would keep them at the space station until next February.
The test pilots anticipated being away just a week or so when they rocketed away as Starliner's first crew.
But thruster failures and helium leaks marred the capsule's trip to the space station, raising doubts about its ability to return safely and leaving the astronauts in limbo.
NASA officials on Wednesday (early on Thursday AEST) said they were analysing more data before making a decision by the end of next week or beginning of the next.
These thrusters are crucial for holding the capsule in the right position when it comes time to descend from orbit.
"We've got time available before we bring Starliner home and we want to use that time wisely," said Ken Bowersox, NASA's space operations mission chief.
Switching to SpaceX would require bumping two of the four astronauts assigned to the next ferry flight, currently targeted for late September. Wilmore and Williams would take the empty seats in SpaceX's Dragon capsule once that half-year mission ends.
Another complication: The space station has just two parking places for US capsules. Boeing's capsule would have to depart ahead of the arrival of SpaceX's Dragon in order to free up a spot.
NASA would like to keep SpaceX's current crew up there until the replacements arrive, barring an emergency.
Those four should have returned to Earth this month, but saw a seventh month added to their mission because of the uncertainty over Starliner, keeping them up there until the end of September. Most space station stays last six months, although some have gone a full year.
Eager to have competing services and backup options, NASA hired SpaceX and Boeing to transport astronauts to and from the space station after the shuttles retired in 2011.
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SpaceX's first astronaut flight was in 2020. Boeing suffered so much trouble on its initial test flight without a crew in 2019 that a do-over was ordered.
Then more problems cropped up, costing the company more than $US1 billion ($1.5 billion) to fix before finally flying astronauts.