Ground controllers tried to plug it the way they handled previous, smaller leaks: stopping and restarting the flow of super-cold liquid hydrogen in hopes of closing the gap around a seal in the supply line. They tried that twice, in fact, and also flushed helium through the line. But the leak persisted.

Blackwell-Thompson finally halted the countdown after three to four hours of futile efforts.

Mission manager Mike Sarafin told journalists it was too early to tell what caused the leak, but it may have been due to inadvertent over-pressurisation of the hydrogen line earlier in the morning when someone sent commands to the wrong valve.

“This was not a manageable leak,” Sarafin said, adding that the escaping hydrogen exceeded flammability limits by two or three times.

During Monday’s attempt, a series of small hydrogen leaks popped up there and elsewhere on the rocket. Technicians tightened up the fittings over the following days, but Blackwell-Thompson had cautioned that she wouldn’t know whether everything was tight until Saturday’s fuelling.

Hydrogen molecules are exceedingly small – the smallest in existence – and even the tiniest gap or crevice can provide a way out. NASA’s space shuttles, now retired, were plagued by hydrogen leaks. The new moon rocket uses the same type of main engines.

Even more of a problem on Monday was that a sensor indicated one of the rocket’s four engines was too warm, though engineers later verified it actually was cool enough. The launch team planned to ignore the faulty sensor this time around and rely on other instruments to ensure each main engine was properly chilled. But the countdown never got that far.

Thousands of people who jammed the coast over the long Labor Day weekend, hoping to see the Space Launch System rocket soar, left disappointed.

The $US4.1 billion test flight is the first step in NASA’s Artemis program of renewed lunar exploration, named after the twin sister of Apollo in Greek mythology.

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Years behind schedule and billions over budget, Artemis aims to establish a sustained human presence on the moon, with crews eventually spending weeks at a time there. It’s considered a training ground for Mars.

Twelve astronauts walked on the moon during the Apollo program, the last time in 1972.

AP