KENNEDY SPACE CENTER, Fla. — For the second time in a week, NASA officials on Saturday called off the test launch of a rocket that is to one day carry astronauts to the moon. It was another setback for a signature national spaceflight program, though NASA officials expressed confidence that would it be just a temporary one.
But top NASA officials stood behind their decision to call off Saturday’s launch, and said that they were ready to wait longer, perhaps trying again later this month or in October, after the cause of a hydrogen leak is understood and resolved.
“The cost of two scrubs is a lot less than a failure,” Bill Nelson, the NASA administrator, said during a news conference on Saturday afternoon.
Though 322 feet tall, NASA’s new rocket is not literally too big to fail. But in terms of the vehicle’s importance to the space agency’s moon plans, it perhaps is.
NASA has already spent more than $40 billion to develop the rocket, known as the Space Launch System, and the capsule, known as Orion. The program is years behind schedule and billions of dollars over budget. And it has faced criticism from supporters of a more commercial approach to spaceflight, who say companies like Elon Musk’s SpaceX offer the most cost-effective and efficient way to advance human journeys to space.
Because NASA has so much invested in this one rocket, a catastrophic failure would delay the moon program by years and perhaps lead to questioning of its value.
Even people who are not fans of the Space Launch System said NASA’s caution is prudent.
“They’re not going to launch prematurely,” said Lori Garver, a former NASA deputy administrator during the Obama administration who has said the rocket is too expensive and prefers commercial approaches to spaceflight. “I don’t worry about that.”
The moon landings half a century ago were part of NASA’s Apollo program. The new return-to -the-moon effort has been named Artemis. In Greek mythology, Artemis was the twin sister of Apollo.
The scrubbed launch is for Artemis I, a weekslong, uncrewed mission that will test the rocket and the capsule where future astronauts will ride. The next Artemis mission, currently scheduled for 2024, will have astronauts aboard, and the third Artemis mission is to land astronauts near the moon’s south pole.
As the countdown clock ticked toward a launch on Saturday, a hydrogen leak was detected in a connector along the hydrogen fuel line leading to the rocket.
“We know that when you get above roughly a 4 percent concentration of hydrogen in ambient air, you’re at risk of having a flammability event,” said Mike Sarafin, the Artemis mission manager.
For this leak, which Mr. Sarafin described as large, the concentrations were two to three times the 4 percent limit. After three attempts to seal the leak failed, the launch attempt was called off at 11:17 a.m. Eastern time by the launch director, Charlie Blackwell-Thompson.
Mr. Sarafin said that the problem might have been related to an incorrect command sent to the propellant-loading system on the launchpad, causing overly high pressures — 60 pounds per square inch instead of 20 — in the fuel line for a few seconds. That could have damaged the gasket in the connector.
A hydrogen leak appeared at the same connector during the first launch attempt on Monday, but it was smaller, and engineers figured out how to keep the hydrogen concentration under 4 percent, and they were able to fill up the rocket with 537,000 gallons of ultracold liquid hydrogen. Monday’s launch was called off when a faulty sensor reported that one of the rocket’s four core-stage engines was not sufficiently chilled.
After Saturday’s launch was called off, NASA officials considered options for what to do next. One was to simply disconnect and reconnect the fuel line and attempt to launch again in a couple of days. “But our confidence level, given the size of the leak that we saw today, was fairly low that that would solve the problem,” Mr. Sarafin said.
The mission managers decided the gasket would be need to be replaced, and engineers are considering whether it would be better to do that work at the launchpad, where they could then run liquid hydrogen through the line to ensure the fix was successful, or first roll the rocket back to a behemoth structure known as the Vehicle Assembly Building. It would be easier to perform the repair work there, but engineers would not be able to test the line with liquid hydrogen until the rocket returned to the launchpad.
Jim Free, an associate administrator at NASA, said in a tweet that he and others at the agency were “disappointed at the outcome but proud of our team for constantly bringing solutions forward.”
While Ms. Garver said the launch teams did the right thing by calling off the launch both times, she wondered about the design of the Space Launch System, which uses largely the same engines and solid rocket boosters that powered the space shuttles — technology that dates back more than half a century.
“The choice to use shuttle engines did lock in hydrogen, and we know hydrogen is leaky,” she said. “Those are all design decisions that, if they continue to bite us, will be of concern.”
But, she added, “Assuming they can get over it in the next round. I think it will be forgotten.”
Even some of the disappointed spectators seemed to understand.
It was the second time in days that throngs of people along Central Florida’s waterways and beaches faced disappointment at missing their chance to take in the first launch of the most powerful rocket since the Saturn V took astronauts to the moon in the 1960s and 1970s.
Last Monday, Vincent Anderson, 45, of Lake Alfred, Fla., took a boat tour with his son in the hopes of seeing the rocket launch. It was not to be.
To his 10-year-old, he said: “Rockets are finicky like cats, they go up when they want to.”
Then the scene replayed again this morning, when Mr. Anderson had signed up for another boat tour, this time with his 15-year-old daughter. He called the scrub “bittersweet” but hedged that they had begun the day with “the same expectations of ‘probably won’t happen.’”
The launch again did not happen, but the outings were still worth it, he said.
Christine Chung contributed reporting.