NASA hit by a fuel leak during a practice countdown of the moon rocket that will fly with astronauts

But just a couple hours into the daylong operation, excessive hydrogen was detected near the bottom of the rocket. Hydrogen loading was temporarily halted, with just half of the core stage filled.

The launch team scrambled to work around the problem using techniques developed during the only other Space Launch System rocket launch three years ago. That first test flight was plagued by hydrogen leaks before finally soaring.

The crew, three Americans and one Canadian, monitored the critical dress rehearsal from nearly 1,000 miles (1,600 kilometers) away in Houston, home to Johnson Space Center. They have been in quarantine for the past 1½ weeks, awaiting the practice countdown's outcome.

The all-day operation will determine when they can blast off on the first lunar voyage by a crew in more than half a century.

Running two days behind because of a bitter cold snap, NASA set its countdown clocks to stop a half-minute before reaching zero, just before engine ignition. The clocks began ticking Saturday night, giving launch controllers the chance to go through all the motions and deal with any lingering rocket problems. Hydrogen leaks kept the first SLS rocket on the pad for months in 2022.

If the fueling demo can be completed successfully on time, NASA could launch commander Reid Wiseman and his crew to the moon as soon as Sunday. The rocket must be flying by Feb. 11 or the mission will be called off until March. The space agency only has a few days in any given month to launch the rocket, and the extreme cold already has shortened February’s launch window by two days.

The nearly 10-day mission will send the astronauts past the moon, around the mysterious far side and then straight back to Earth, with the goal of testing the capsule's life support and other vital systems. The crew will not go into lunar orbit or attempt to land.

NASA last sent astronauts to the moon during the 1960s and 1970s Apollo program. The new Artemis program aims for a more sustained lunar presence, with Wiseman’s crew setting the stage for future moon landings by other astronauts.

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