Barry 'Butch' Wilmore and Sunita 'Suni' Williams were originally scheduled to spend a little more than a week aboard the ISS
Barry 'Butch' Wilmore and Sunita 'Suni' Williams were originally scheduled to spend a little more than a week aboard the ISS AFP

NASA needs to decide by the end of August whether to return two astronauts to Earth aboard Boeing's Starliner, which flew them to the International Space Station (ISS), or bring them home on a rival SpaceX craft, officials said Wednesday.

NASA astronauts Barry "Butch" Wilmore and Sunita "Suni" Williams blasted off for the ISS on Starliner on June 5 for what was meant to be an eight-day stay.

But their return has been delayed by thruster malfunctions that came to light during the first crewed mission to the ISS by the Boeing spacecraft.

NASA officials, at a press conference on Wednesday, said they were still analysing thruster data, but a decision on whether to use Starliner or SpaceX was looming.

"We don't have a sharp cutoff, right?" said Ken Bowersox, associate administrator of NASA's Space Operations Mission Directorate. "We can juggle things and make things work if we need to extend.

"But it's getting a lot harder," Bowersox said. "We're reaching a point where that last week in August we really should be making a call, if not sooner."

He said the two astronauts were "making the best" of their extra time aboard the ISS "but I'm sure they're eager for a decision, just like the rest of us."

Bowersox said the main issue is with the propulsion system. "Our big concern is having a successful deorbit burn," he said.

The NASA official said there have been "very honest discussions" with Boeing and the company is "100 percent behind their vehicle."

"They have confidence in their vehicle," he said.

In the event a decision is made not to use Starliner, Boeing's rival SpaceX, owned by Elon Musk, could potentially launch their scheduled Crew-9 mission to the ISS on September 24 with just two astronauts rather than the usual four.

The Crew Dragon capsule would then be able to return to Earth with Wilmore and Williams in February 2025 -- in what would amount to a major embarrassment for aerospace giant Boeing.

NASA chief astronaut Joe Acaba said Wilmore and Williams prepared for the mission knowing that it was a test flight and it "might not be perfect."

"Human space flight is inherently risky and as astronauts we accept that as part of the job," he said.