• The problem was traced to the connection where the hydrogen was being pumped into the vehicle.
• Controllers tried a number of fixes, including allowing the hardware to warm up for short periods, hoping this might reset the seal. But without success.
Image: NASA
The US space agency has had to postpone the launch of its new Artemis I Moon rocket for the second time in a week.
Controllers were unable to stop a hydrogen leak on the vehicle, almost from the start of Saturday's countdown procedure.
Nasa now has another opportunity to launch the rocket on Monday or Tuesday.
After that the vehicle will have to return to its assembly building for inspection and maintenance, which will mean further delays.
The Space Launch System is the most powerful rocket ever developed by Nasa, and is designed to send astronauts and their equipment back to the lunar surface after an absence of 50 years.
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Much of the SLS's enormous thrust comes from burning almost three million litres of super-cold liquid hydrogen and oxygen in four big engines on the vehicle's underside.
But when controllers sent the early morning command to fill the rocket's hydrogen tank, an alarm went off, indicating there was a leak.
The problem was traced to the connection where the hydrogen was being pumped into the vehicle.
Controllers tried a number of fixes, including allowing the hardware to warm up for short periods, hoping this might reset the seal. But without success.
The Artemis I mission is an uncrewed demonstration, but Nasa Administrator Bill Nelson said the rocket's future role in human spaceflight meant extreme care was still required in its operation.
"We will go when it's ready," he stressed. "We don't go until then, and we make sure it's right before we put humans up on the top of it."
It's possible Nasa could try again in the next few days. But there are battery systems on this rocket that will soon need inspection. And if the vehicle has to be rolled back to the engineering building for further work, it could be mid-October before we see it again on the launch pad.
Nasa astronaut Zena Cardman said she understood everyone's frustration but they should recognise the challenge of working with super-cold propellants.
"Cryogenic fuels like this are really difficult to handle and so part of the process is just making sure they get into the rocket correctly. This issue is not dissimilar to what we saw the other day. But we just ran out of time to troubleshoot it before the launch window closed," she told BBC News.
Saturday's attempt to despatch the Space Launch System (SLS) rocket had been scheduled for the start of a two-hour window beginning at 14:17 local time (19:17 BST; 18:17 GMT).
The 100m-tall vehicle's objective is to hurl a human-rated capsule, called Orion, in the direction of the Moon, something that hasn't happened since Project Apollo ended in 1972.