Nasa has called off the launch of its big new Moon rocket – the Space Launch System (SLS).
Controllers struggled to get an engine on the 100m-tall vehicle cooled down to its correct operating temperature.
They had previously worried about what appeared to be a crack high up on the rocket but eventually determined it was merely frost build-up.
The SLS is the biggest rocket ever developed by Nasa. It will be used to send astronauts back to the Moon.
The maiden flight, part of Nasa’s Artemis programme, is just a demonstration with no one on board. But ever more complex missions are planned for the future that will see people live on the lunar surface for weeks at a time.
Nasa has the option to try again on Friday, if the engine issue can be resolved by then.
But it’s possible the rocket may have to be rolled back to the assembly building at the Kennedy Space Center for more extensive work.
This could see the flight delayed until September.