The inaugural launch of NASA’s Space Launch System, a powerful and complex rocket built to carry astronauts to the moon and beyond, won’t be getting off the ground just yet. On Saturday, NASA’s team of engineers at Kennedy Space Center in Florida decided to hold off for more repairs and troubleshooting, following a hydrogen leak discovered during liftoff preparations on the launch pad. It’s not yet clear what the repairs will entail, nor if they can be accomplished on the pad or will require rolling the rocket back to the Vehicle Assembly Building. Either way, it will delay the start of the Artemis lunar program until a new launch period opens at the end of September, or the following one does in late October. It’s a significant postponement for the uncrewed spaceflight, intended to fly in a lunar orbit before returning to Earth. But since it’s a precursor to sending astronauts to the moon—for the first time since the Apollo program—there’s a lot at stake. “We’re going to make sure it’s right before we put four humans up on top of it,” said NASA chief Bill Nelson on the space agency’s livestream following the launch scrub. He added that, in the space business, such challenges come with the territory, especially with new spacecraft. “This is part of our space program. Be ready for the scrubs. ” The problem today came from a leak in a line used to load liquid hydrogen fuel into the core-stage SLS rocket, posing a flammability hazard. The team had encountered a smaller leak during Monday’s launch attempt, but today’s “was not a manageable leak,” said Mike Sarafin, the Artemis mission manager, at a press conference late Saturday afternoon. “The team tried three times to resolve it; all three times they saw a large leak. ” NASA plans to use the SLS rocket design, with some variations in capabilities, for the entire Artemis program. It is topped by the Orion crew capsule. But the rocket, built by contractors Boeing, Northrop Grumman, and Aerojet Rocketdyne, has already had a rough ride. Its development had so many delays and budget overruns that at one point it seemed doomed . A “wet dress rehearsal” test in April , during which NASA engineers tested fueling and running the countdown sequence, sent the rocket back for some repairs . The team got closer in June , when they were able to fully load the propellant and practice the countdown to within 29 seconds of launch—but they discovered a faulty helium check valve and a liquid hydrogen leak. After making these fixes, they returned the rocket to the launch pad on August 18, and then four days later declared that the SLS had passed its flight readiness review, scheduling the first launch attempt for August 29. Yet that try on Monday didn’t go as planned . While loading the core stage with propellants, NASA engineers noticed a problem with the third RS-25 engine, one of the engines next to the right solid rocket booster. The flow of liquid hydrogen into the engine’s compartment wasn’t working as it should, and the propellant wasn’t at the proper temperature range. (This issue, called a hydrogen kickstart test, was on their checklist during the June wet dress rehearsal, but the engineers were unable to test it then because of a liquid hydrogen leak. ) The team also spotted a crack in the insulating foam on the outside of the rocket, but they determined that that didn’t pose a significant risk. NASA engineers held the countdown at T-40 minutes while troubleshooting for more than an hour. Finally, launch director Charlie Blackwell-Thompson called the attempt a scrub . At a press conference the following day, members of the Artemis team suggested the apparent engine issue might actually have been a sign of a dodgy temperature sensor. “The way the sensor is behaving does not line up with the physics of the situation,” said John Honeycutt, the SLS program manager. The launch was then pushed back to this weekend, with countdown procedures starting up again early Saturday morning. Anticipating challenges with the propellants, they began the chilldown process, including the kickstart test, about 45 minutes earlier during the countdown procedures. The launch team and weather officer confirmed that the weather was amenable to launch, despite a few intermittent rain showers. They began filling the big orange fuel tank with more than 700,000 gallons of liquid hydrogen and liquid oxygen, supercooled to a frigid -423 and -297 degrees Fahrenheit. But that’s when the hydrogen leak arose, after the oxygen had been mostly fueled up. “Hydrogen's difficult to work with,” said Jim Free, as associate administrator at NASA headquarters, during the post-scrub press conference. The leak seems to stem from a seal in the eight-inch quick disconnect, a fitting used for the liquid hydrogen supply line from the ground system. Eventually, it became clear that that fitting would have to be removed and replaced. At 11:17 am Eastern time, Blackwell-Thompson made the call to scrub the launch attempt. In an industry where “space is hard” is a cliché, such delays aren’t out of the ordinary, even when the weather cooperates. During NASA’s space shuttle program, some ultimately successful launches had to be postponed multiple times. With the SLS—a huge, brand-new rocket with numerous systems to coordinate—the task becomes even more formidable. NASA has 489 “launch commit criteria” that have to be met before they can be “go” for launch, Sarafin said at a press conference on September 1. NASA may need to delay the Artemis launch until mid-October, to come after SpaceX’s Crew-5 launch at a neighboring pad—which has been postponed multiple times, too. That mission will bring two NASA astronauts, a Japanese astronaut and a Russian cosmonaut, Anna Kikina, to the International Space Station. This will be the first time a Russian will fly aboard a US-made spacecraft since the conflict in Ukraine led to tensions between Roscosmos, NASA, and other space agencies. The team is still considering whether repairs can be made on the launch pad, or if the rocket must be rolled back to the Vehicle Assembly Building. “There’s a risk versus risk tradeoff,” said Sarafin, noting that keeping the rocket on the pad exposes it to environmental risks, but that the quick disconnect seal cannot be tested at cryogenic temperatures inside the building. A rollback itself is not without risks, since the motion and vibrations can put stress on the rocket. But to minimize wear and tear, the rocket would move no faster than one mile per hour on a machine called “ the crawler . ” That rollback option would ensure a delay until late October, which could also pose risks for the small spacecraft aboard the rocket , intended for their own mini missions. Those spacecraft, called CubeSats, have batteries with limited power—some of them can be recharged, but others can’t. “If we need to roll back to the Vehicle Assembly Building, we can top off the batteries for a number of those,” Sarafin said at the press conference. “It is part of the process of looking at a given launch period. ” Nelson emphasized that Artemis 1 is a test flight, and said that today’s pushback is not expected to affect the overall timeline for the program, which aims to send astronauts into lunar orbit aboard Artemis 2 in 2024, and to land them on the moon aboard Artemis 3 in 2025. (That moon landing mission may slip to 2026, however, according to a March assessment by the NASA Inspector General . ) While the Artemis team wanted to launch today, NASA officials stressed that the rocket is in good condition, and that they’re confident that they’ll be able to launch safely in the near future. “We’re not where we want to be, except the vehicle is safe—it's not safe in orbit, it's safe on the ground,” Free said. .