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.
The scrub will have disappointed the hundreds of thousands of spectators who had gathered on local beaches and causeways to see the most powerful rocket in 50 years fly skyward.
But Nasa Administrator Bill Nelson, himself a one-time astronaut, said the cautious approach was the right one.
“We don’t launch until it’s right,” he stressed. “And I think it’s just illustrative that this is a very complicated machine, a very complicated system. And all those things have to work. And you don’t want to light the candle until it’s ready to go.”
Nasa has the option to try again on Friday, if the engine issue can be resolved by then.
Worryingly, this particular power unit also played up during a recent countdown rehearsal.
If controllers have to roll the rocket back to Kennedy’s assembly building to swap out the engine, it will introduce several weeks’ delay.
Nasa also has to be mindful of the weather. Conditions here in Florida are very dynamic at this time of year. Electrical storms frequently pass over the spaceport. It’s best to try to launch in the morning. It’s generally calmer. But the opportunity this Friday and on Monday next are afternoon launch windows.
When it eventually lifts off, the rocket’s job will be to propel a test capsule, called Orion, far from the Earth.
This spacecraft will loop around the Moon on a big arc before returning home to a splashdown in the Pacific Ocean six weeks later.
Orion will be uncrewed for the first mission, but – assuming all the hardware works as it should – astronauts will climb aboard for a future series of missions, starting in 2024.
The current mission’s chief objective actually comes right at the end of the 42-day flight. (BBC)