Nasa has suspended the launch of its big new moon rocket – the Space Launch System (SLS).
Testers struggled to get an engine in the 100-meter-tall vehicle to cool to its proper operating temperature.
They had previously been concerned about what appeared to be a crack high up in the rocket, but ultimately decided it was just frost buildup.
SLS is the largest rocket ever developed by Nasa. It will be used to send astronauts back to the Moon.
The maiden flight, part of Nasa’s Artemis program, is just a demonstration with no one on board. But increasingly complex missions are planned for the future that will see humans living on the lunar surface for weeks at a time.
The scrub will have disappointed him hundreds of thousands of viewers who had gathered on local beaches and trails to watch the most powerful rocket in 50 years soar into the sky.
But Nasa administrator Bill Nelson, who was once an 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 indicative that this is a very complex machine, a very complex system. And it all has to work. And you don’t want to light the candle until it’s ready.”
Nasa has the option to try again on Friday if the engine problem can be resolved by then.
Worryingly, this particular power unit also performed during a recent countdown rehearsal.
If controllers have to roll the rocket back to the Kennedy assembly building to swap the engine, there will be a delay of several weeks.
Nasa also has to watch the weather. Conditions here in Florida are very dynamic this time of year. Electrical storms often pass over the spaceport. It is best to try to start in the morning. It’s generally calmer. But the opportunity this Friday and next Monday are afternoon launch windows.
When it finally takes off, the rocket’s job will be to propel a test capsule, called Orion, away from Earth.
This spacecraft will wander around the Moon in a large arc before returning home in a dive in the Pacific Ocean six weeks later.
Orion will be undocked for the first mission, but — assuming all the hardware is working as it should — astronauts will board the craft for a future series of missions, starting in 2024.
The main objective of the current mission actually comes right at the end of the 42-day flight.
Engineers are most concerned to see that Orion’s heat shield will handle the extreme temperatures it will encounter upon re-entry into Earth’s atmosphere.
Orion will come very fast – at 38,000 km/h (24,000 mph), or 32 times the speed of sound.
“Even the reinforced carbon that protected the shuttle was only good for about 3,000 F (1,600 C),” said Mike Hawes, Orion program manager at aerospace company Lockheed Martin.
“Now, we’re coming up to over 4,000 F (2,200 C). We’ve gone back to the Apollo removal material called Avcoat. It’s in vacuum-filled blocks, and testing is a high priority.”
The European Space Agency provided the service module for Orion. This is the rear section that propels the capsule into space. It’s a contribution of the kind that Europe hopes will lead to the inclusion of its nationals in future trips to the lunar surface.
Several missions are planned – right now it’s up to Artemis IX.
At this stage there should be habitats and rovers on the Moon for use by astronauts.
But ultimately, Artemis is seen as a proving ground for getting people to Mars.
“The timeline for that was set by President Obama. He said 2033,” Mr. Nelson recalled.
“Every successive administration has supported the program, and the realistic timeline I’m now informed of is the late 2030s, maybe even the 2040s.”