CAPE CANAVERAL, Fla. (AP) — A fuel leak interrupted NASA’s launch countdown for its new moon rocket early Monday, reappearing in the same place that saw seepage during a dress rehearsal back in the spring.

Launch controllers halted the tanking operation, which already was running an hour late because of thunderstorms offshore. They slowly resumed the process to confirm that it was, indeed, a hydrogen fuel leak and not faulty sensors, but alarms forced another temporary pause as precious minutes in the countdown ticked away.

The 322-foot (98-meter) rocket is the most powerful ever built by NASA, out-muscling even the Saturn V that carried astronauts to the moon a half-century ago.

This test flight, if successful, would put a crew capsule into lunar orbit for the first time in 50 years.

No astronauts were inside the Orion capsule atop the rocket at NASA’s Kennedy Space Center. Instead, three test dummies were strapped in for the lunar-orbiting mission, expected to last six weeks.

Even with no one on board, thousands of people jammed the coast to see the Space Launch System, or SLS, rocket soar. Vice President Kamala Harris flew into Orlando with her husband, but had yet to make the hourlong drive to Cape Canaveral for the planned liftoff.

The next launch attempt wouldn’t be until Friday at the earliest.

Hydrogen fuel leaks marred NASA’s countdown test back in April, prompting a slew of repairs. The demo was repeated with more success in June, but that, too, experienced some leakage. Managers said they would not know for certain whether the fixes were good until attempting to load the rocket’s tanks with nearly 1 million gallons of super-cold fuel on Monday.

Launch director Charlie Blackwell-Thompson and her team also had to deal with a communication issue involving the Orion capsule.

Engineers scrambled to understand an 11-minute delay in the communication lines between Launch Control and Orion that cropped up late Sunday. Although the problem had cleared by Monday morning, NASA needed to know why it occurred before committing to launch.

This first flight of NASA’s 21st-century moon-exploration program, named Artemis after Apollo’s mythological twin sister, is years overdue. Repeated delays have led to billions in budget overruns; this demo alone costs $4.1 billion.

Assuming the test goes well, astronauts would climb aboard for the second flight and fly around the moon and back as soon as 2024. A two-person lunar landing could follow by the end of 2025. NASA is targeting the moon’s south pole.

During Apollo, 12 astronauts landed on the moon from 1969 through 1972, with stays of no more than a few days. NASA is looking to establish a lunar base during Artemis, with astronauts rotating in and out for weeks at a time. The next step would be Mars, possibly in the late 2030s or early 2040s.

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