Update, 11:28 a.m. EDT: The Artemis 1 mission has been scrubbed due to a fuel leak. The next possible opportunity is Monday (Sept. 5).
CAPE CANAVERAL, Fla. — NASA engineers are working to stem a fuel leak on its massive new moon rocket ahead of a critical test flight on Saturday (Sept. 3).
NASA began fueling the Artemis 1 moon rocket, NASA's first Space Launch System (SLS), early Saturday to launch an uncrewed Orion spacecraft to the moon from Pad 39B here at the Kennedy Space Center at 2:17 p.m. EDT (1817 GMT). But a persistent liquid hydrogen fuel leak detected has slowed launch preparations.
NASA has a two-hour window in which to launch the mission, with promising weather forecast varying between 60% and 80% "go," the agency has said. You can watch it launch live online.
Related: NASA's Artemis 1 moon mission: Live updates
More: 10 wild facts about the Artemis 1 moon mission
The hydrogen leak was first detected at 7:15 a.m. EDT (1115 GMT) as NASA began filling the massive SLS rocket with the 730,000 gallons of liquid hydrogen and liquid oxygen propellant needed for launch. The leak is in the connection point of an 8-inch hydrogen fuel line near the engine compartment at the base of the 32-story rocket.
Engineers first tried to warm the connector and chill it with cold fuel to stem the leak, then tried to repressurize it with helium to solve it. Both attempts failed, with NASA trying that first warm-and-chill method again to stop the leak.
A glitch-free fueling process for Artemis 1 can take up to four hours, But there's not much room in the schedule for delays, NASA has said.
It was during the fueling process (or "tanking" as NASA calls it) that launch controllers hit a snag during the agency's first launch attempt on Monday (Aug. 29). NASA called off that launch try when it could not confirm that one of the four RS-25 main engines of the SLS core stage was at its proper chilled temperature of minus 420 degrees Fahrenheit for launch.
The agency ultimately determined that a bad sensor caused the issue and implemented workarounds for Saturday's launch attempt. On Saturday, NASA planned to begin the main engine chill down process earlier to allow more time for the SLS rocket's engines to reach their target temperature. But that process has been delayed by the fuel leak.
NASA's Artemis 1 mission is the first test flight for NASA's Artemis program to return astronauts to the moon by 2025. The mission is a 37-day trip around the moon to verify the Orion spacecraft and SLS rocket are safe for astronauts. The mission will also deploy 10 Artemis cubesats on the way to the moon and carries a series of experiments to measure the astronaut experience on the Orion spacecraft.
Artemis 1 is scheduled to return to Earth on Oct. 11 with a splashdown in the Pacific Ocean off the coast of California. If all goes well, NASA aims to launch a crew around the moon on the Artemis 2 mission in 2024, followed by the Artemis 3 crewed lunar landing in 2025.
Editor's note: This story, originally posted at 7 a.m. ET, has been updated to include details of the fuel leak NASA is working to stop on the SLS rocket. Follow our Artemis 1 mission live updates page for the latest on Artemis 1 mission news. Visit Space.com for live webcast.
Email Tariq Malik at tmalik@space.com or follow him @tariqjmalik. Follow us @Spacedotcom, Facebook and Instagram.
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