CAPE CANAVERAL, Fla. — It may be several weeks before NASA can attempt to launch its massive Space Launch System moon rocket after it was unable to control what agency’s officials described as a large, unmanageable hydrogen leak that forced them to cancel a second flight on Saturday.

Agency officials said they believe it is likely they will have to roll the rocket back to its assembly building to make repairs after two unsuccessful attempts to launch it on a maiden test flight that would loft the uncrewed Orion spacecraft to the moon.

The decision follows another day of disappointment for the space agency, which had been hoping finally to launch the rocket after years of delays and setbacks and mark a significant milestone toward return astronauts to the surface of the moon.

Instead, the next launch attempt could come well into October, as NASA struggles to figure out a complicated, fickle beast of a rocket and its nettlesome propellant.

While NASA officials say the scrub is a normal part of spaceflight, especially with a new rocket, the inability of NASA to launch its flagship rocket is sure to renew criticism from some that it is a symbol of government mismanagement influenced by political whims and reliant on antiquated technology.

The rocket is billions of dollars over budget and years behind schedule, and by some estimates, each launch will cost between $2 billion and $4 billion. In creating the rocket, Congress dictated that it recycle engines and technology from the space shuttle program, which first flew in 1981 and was developed in the 1970s.

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Unlike the rockets used by SpaceX to launch astronauts to the International Space Station, which return to Earth to be used again, the Space Launch System is not reusable.

Officials hoped getting the rocket to the pad for its first launch would make a statement that NASA had revived its deep-space ambitions. But instead of celebrating a triumphant flight that would put it on a path to the moon, NASA officials spent most of the morning Saturday scrambling trying to fix a leak of the volatile liquid hydrogen used as the rocket’s fuel.

Hydrogen, the lightest element, is kept in liquid form at minus 423 degrees Fahrenheit, and NASA has had a difficult time loading it into the rocket’s tanks without it leaking.

NASA encountered a similar problem during a launch attempt on Monday but was able to eventually overcome it. On Saturday, though, engineers started loading the hydrogen, then stopped at about 7:15 a.m. Then they started again but had to stop at about 9 a.m. after it started leaking again. They tried to warm the line, then use helium to pressurize it, but neither worked.

Nor did a second attempt at warming the line.

At 11:17 a.m., three hours before the launch window even opened, NASA called a scrub. The leak on was larger than the one they encountered Monday, said Mike Sarafin, the Artemis program manager. “The leak on Monday was a manageable leak,” he said. “This was not a manageable leak.” He said one of the hydrogen lines was inadvertently over pressurized, but it was unclear whether what caused the leak or exactly why the overpressurization had occurred.

Officials said they are considering whether they can repair the leak on the launchpad and then test it there by flowing the liquid hydrogen through it — a test they could not do if the rocket is taken back to the assembly building. If the repair cannot be done on the pad, they would have to roll it back to the assembly building and make the repairs there, but could not be certain the problem had been fixed till another launch try.

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Either way, engineers will need to reset the rocket’s emergency flight termination system, which destroys the rocket in case it goes wildly off course during launch. That work can only be done in the assembly building.

Whether for technical reasons or bad weather, delays are nothing new to the space program. Officials noted Saturday that out of 135 space shuttle launches, 121 were scrubbed at least once. In 20 cases, the spacecraft were returned to the assembly building.

On Saturday, NASA Administrator Bill Nelson praised the SLS’s launch team, saying that the agency will be prudent and won’t rush the launch until it has all the systems working.

“We don’t go until then and especially now on a test flight,” Nelson said. NASA will “make sure it’s right before we put four humans on top of it.” Delays, he said, are “part of the space business.” He noted that scrubs are far less costly than a failure.

As a member of Congress, Nelson flew on the space shuttle in 1986, but had to endure repeated delays.

“We scrubbed four times,” he said. “We were delayed over the better part of a month … This is part of our space program: be ready for the scrubs.”

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When she was an astronaut preparing to fly on the space shuttle, Pam Melroy, who now serves as the deputy NASA administrator, used to tell her friends and family to plan a week’s vacation on the Florida Space Coast “and maybe you’ll see a launch.”

The Artemis I mission, as it is called, has no astronauts on board and is a test to ensure that the rocket and spacecraft are safe for humans to ride. If NASA is able to complete Artemis I, the next flight will put four astronauts on board for a flight around the moon, perhaps in 2024. A human landing on the lunar surface could come in 2025 or 2026.

But as Saturday’s setback shows, NASA still has many technical challenges to overcome. NASA is being particularly cautious with its SLS rocket. It has cost some $23 billion to develop, and the space agency hopes it will serve as the backbone of its Artemis program, designed to return astronauts to the moon.

Sarafin, the Artemis mission manager, told reporters this week that there are nearly 500 launch criteria that have to be met and that any number of things could force the space agency to scrub and try another day.

“There’s no guarantee that we’re going to get off,” he said. “But we’re going to show up, and we’re going to try, and we’re going to give it our best.”

Saturday’s scrub followed one from Monday, when engineers said they were unable to get one of the four engines mounted to the booster stage to the right temperature required for launch. After standing down and investigating the problem, they determined that a faulty sensor was to blame, and moved to try again Saturday.

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Last year, NASA was able to successfully load the rocket with more than 730,000 gallons of liquid hydrogen and liquid oxygen and test fire the RS-25 engines for their full eight-minute duration. But it has been unable to repeat that success since then.

Earlier this year, in a series of fueling tests, it ran into all sorts of problems that prevented the space agency for running a full simulated countdown.

Despite those problems, NASA officials said they felt confident enough to proceed to a launch attempt. The attempts have essentially served as additional fueling tests.

Victor Glover, a NASA astronaut who could be among one of the crews chosen for an Artemis moon mission, said that the flight controllers should be praised for their decision to cancel the launch, especially how much public attention it received.

“It’s hard to make a decision like this,” he said. “We can be mad at the hydrogen leak, but the scrub is absolutely the right call, and it helps to build trust.”