Propellant leak delays SpaceX launch of private Ax-4 astronaut mission to the ISS

Four private astronauts will have to wait a bit longer to get to space.

The four-person Ax-4 mission to the International Space Station (ISS) was supposed to lift off atop a SpaceX Falcon 9 rocket from NASA's Kennedy Space Center in Florida on Wednesday morning (June 11). But that's no longer the plan.

"Standing down from tomorrow’s Falcon 9 launch of Ax-4 to the @Space_Station to allow additional time for SpaceX teams to repair the LOx leak identified during post static fire booster inspections. Once complete — and pending Range availability — we will share a new launch date," SpaceX announced via X on Tuesday evening (June 10).

LOx is liquid oxygen, one of the two propellants used by the Falcon 9's Merlin engines. The other is RP-1, a rocket-grade kerosene.

As SpaceX's post mentioned, the company noticed the LOx leak after static-firing the rocket's first stage, an activity that took place on Sunday (June 8). Static fires are common prelaunch tests in which a rocket's engines are briefly ignited while the booster is anchored to the pad.

Bill Gerstenmaier, SpaceX's vice president of build and flight reliability, discussed the leak during a press conference on Monday (June 9) that was held after Ax-4's launch readiness review.

This Falcon 9 first stage has one flight under its belt, and the leak was also seen on that mission, during its reentry to Earth's atmosphere, Gerstenmaier said.

SpaceX "discovered that we had not fully repaired the booster during refurbishment, or we didn't, actually, didn't find the leak and didn't get it corrected," he said.

Gerstenmaier said on Monday that SpaceX was still troubleshooting the leak, though he expressed confidence that the issue wouldn't prevent an on-time liftoff.

"We're installing a purge that will essentially mitigate the leak, if it still continues, if we see it on launch day," he said. "So we will be fully ready to go fly."

Ax-4 will be the fourth astronaut mission to the ISS organized by the Houston company Axiom Space. It will be commanded by Peggy Whitson, a record-breaking former NASA astronaut who is now Axiom's director of human spaceflight.

The other crewmembers are pilot Shubhanshu Shukla of India; mission specialist Sławosz Uznański-Wiśniewski of Poland and the European Space Agency; and Hungarian mission specialist Tibor Kapu. No astronaut from any of these three countries has ever visited the ISS before.

The Ax-4 astronauts will spend about two weeks living and working aboard the orbiting lab, performing about 60 different science experiments. They'll then return to Earth with a splashdown in the Pacific Ocean.