CAPE CANAVERAL, Fla. -- 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 window to launch the rocket opens at 8:33 a. m. ET. NASA has two hours to launch before the window closes. 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. MORE: Group of visually impaired veterans to experience Artemis 1 launch in a unique way 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. Here's a rundown of the first flight of the Artemis program, named after Apollo's mythological twin sister. The new rocket is shorter and slimmer than the Saturn V rockets that hurled 24 Apollo astronauts to the moon a half-century ago. But it's mightier, packing 8. 8 million pounds (4 million kilograms) of thrust. It's called the Space Launch System rocket, SLS for short, but a less clunky name is under discussion, according to Nelson. Unlike the streamlined Saturn V, the new rocket has a pair of strap-on boosters refashioned from NASA's space shuttles. The boosters will peel away after two minutes, just like the shuttle boosters did, but won't be fished from the Atlantic for reuse. The core stage will keep firing before separating and crashing into the Pacific in pieces. Two hours after liftoff, an upper stage will send the capsule, Orion, racing toward the moon. NASA's high-tech, automated Orion capsule is named after the constellation, among the night sky's brightest. At 11 feet (3 meters) tall, it's roomier than Apollo's capsule, seating four astronauts instead of three. For this test flight, a full-size dummy in an orange flight suit will occupy the commander's seat, rigged with vibration and acceleration sensors. Two other mannequins made of material simulating human tissue - heads and female torsos, but no limbs - will measure cosmic radiation, one of the biggest risks of spaceflight. One torso is testing a protective vest from Israel. Unlike the rocket, Orion has launched before, making two laps around Earth in 2014. This time, the European Space Agency's service module will be attached for propulsion and solar power via four wings. Orion's flight is supposed to last six weeks from its Florida liftoff to Pacific splashdown, twice as long as astronaut trips in order to tax the systems. It will take nearly a week to reach the moon, 240,000 miles away. After whipping closely around the moon, the capsule will enter a distant orbit with a far point of 38,000 miles. That will put Orion 280,000 miles from Earth, farther than Apollo. The big test comes at mission's end, as Orion hits the atmosphere at 25,000 mph on its way to a splashdown in the Pacific. The heat shield uses the same material as the Apollo capsules to withstand reentry temperatures of 5,000 degrees Fahrenheit. But the advanced design anticipates the faster, hotter returns by future Mars crews. Besides three test dummies, the flight has a slew of stowaways for deep space research. Ten shoebox-size satellites will pop off once Orion is hurtling toward the moon. The problem is these so-called CubeSats were installed in the rocket a year ago, and the batteries for half of them couldn't be recharged as the launch kept getting delayed. NASA expects some to fail, given the low-cost, high-risk nature of these mini satellites. The radiation-measuring CubeSats should be OK. Also in the clear: a solar sail demo targeting an asteroid. In a back-to-the-future salute, Orion will carry a few slivers of moon rocks collected by Apollo 11's Neil Armstrong and Buzz Aldrin in 1969, and a bolt from one of their rocket engines, salvaged from the sea a decade ago. Aldrin isn't attending the launch, according to NASA, but three of his former colleagues will be there: Apollo 7's Walter Cunningham, Apollo 10's Tom Stafford and Apollo 17's Harrison Schmitt, the next-to-last man to walk on the moon. More than 50 years later, Apollo still stands as NASA's greatest achievement. Using 1960s technology, NASA took just eight years to go from launching its first astronaut, Alan Shepard, and landing Armstrong and Aldrin on the moon. By contrast, Artemis already has dragged on for more than a decade, despite building on the short-lived moon exploration program Constellation. Twelve Apollo astronauts walked on the moon from 1969 through 1972, staying no longer than three days at a time. For Artemis, NASA will be drawing from a diverse astronaut pool currently numbering 42 and is extending the time crews will spend on the moon to at least a week. The goal is to create a long-term lunar presence that will grease the skids for sending people to Mars. NASA's Nelson, promises to announce the first Artemis moon crews once Orion is back on Earth. There's a lot more to be done before astronauts step on the moon again. A second test flight will send four astronauts around the moon and back, perhaps as early as 2024. A year or so later, NASA aims to send another four up, with two of them touching down at the lunar south pole. Orion doesn't come with its own lunar lander like the Apollo spacecraft did, so NASA has hired Elon Musk's SpaceX to provide its Starship spacecraft for the first Artemis moon landing. Two other private companies are developing moonwalking suits. The sci-fi-looking Starship would link up with Orion at the moon and take a pair of astronauts to the surface and back to the capsule for the ride home. So far, Starship has only soared six miles. Musk wants to launch Starship around Earth on SpaceX's Super Heavy Booster before attempting a moon landing without a crew. One hitch: Starship will need a fill-up at an Earth-orbiting fuel depot, before heading to the moon. ___ The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content. .