Just hours before lift-off, NASA has halted fuelling its new moon rocket for the second time this week.

The launch team began loading fuel into the 322ft rocket on Saturday morning but the operation was halted after a hazardous leak was detected.

Monday’s lift-off attempt, of the most powerful rocket ever built by NASA, was halted by an engine sensor fault and leaking fuel.

What stopped NASA moon rocket fuelling?

As the sun rose at Kennedy Space Centre in Florida on Saturday, an over-pressure alarm sounded and the fuelling operation was briefly halted, but no damage occurred and the effort resumed, Nasa’s Launch Control reported.

But minutes later, hydrogen fuel began leaking from the engine section at the bottom of the rocket. Nasa halted the operation, while engineers scrambled to plug what was believed to be a gap around a seal.

The countdown clocks continued ticking towards an afternoon lift-off – Nasa has two hours from 2.17pm local time (7.17pm in the UK) to get the rocket off.

NASA moon mission

Nasa wants to send the crew capsule on top of the rocket around the moon, pushing it to the limit before astronauts get on the next flight.

If the five-week demo with test dummies succeeds, astronauts could fly around the moon in 2024 and land on it in 2025. People last walked on the moon 50 years ago.

Forecasters expect generally favourable weather at Kennedy Space Centre, especially towards the end of the two-hour launch window.

On Monday, a sensor indicated one of the four engines was too warm, but engineers later verified it actually was cold enough.

The launch team plan to ignore the faulty sensor this time around and rely on other instruments to ensure each main engine is properly chilled.

Before igniting, the main engines need to be as frigid as the liquid hydrogen fuel flowing into them at minus 250C. If not, the resulting damage could lead to an abrupt engine shutdown and aborted flight.

NASA moon mission risks

Mission managers accepted the additional risk posed by the engine issue as well as a separate problem: cracks in the rocket’s insulating foam. But they acknowledged other problems could prompt yet another delay.

That has not stopped thousands flocking to the coast to see the Space Launch System rocket soar. Local authorities expect massive crowds because of the long Labour Day holiday weekend.

The 4.1 billion US dollar (£3.5 billion) test flight is the first step in Nasa’s Artemis programme of renewed lunar exploration, named after the twin sister of Apollo in Greek mythology.

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

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