A SpaceX crew on a rescue mission have arrived at the International Space Station, but it will still be some time before stranded Boeing Starliner astronauts Butch Wilmore and Suni Williams can return back to Earth.

SpaceX’s Falcon 9 rocket is grounded again after the vehicle’s second stage did not come down in the expected area of the ocean, following an otherwise successful mission that delivered a Dragon capsule and its crew to orbit. “We will resume launching once we better understand root cause,” the company said in a statement posted to X .

NASA engineers are running checks to solve the mystery of why the Falcon 9's engine failed to perform a routine maneuver to lower the rocket's altitude.