KENNEDY SPACE CENTER, Fla. — Camille Calibeo woke up at about 2 a.m. to board a boat to get a prime view of the launchpad where NASA’s giant new moon rocket sat.
Thousands of people had come from near and far to pack the beaches, roadsides, rooftops and waterways. Some even camped overnight in hopes of seeing that rocket launch for the first time, rising upward with a thunderous boom and jets of fire from its engines.
Ms. Calibeo, 25, had traveled from Denver. Vice President Kamala Harris flew in from Washington on Air Force Two.
“We are going,” proclaimed NASA banners hung all around the space center.
But on Monday, the rocket did not go, and NASA officials said it was too early to guess whether it might be able to launch Friday, the next potential opportunity, or later. Mission managers will meet on Tuesday to discuss their next steps.
Although there will be no astronauts on this test flight, this rocket — what NASA calls the Space Launch System — is to usher in a new era of human exploration including sending the first woman and the first person of color to the surface of the moon.
The first mission, without astronauts, is to be a weekslong flight around the moon to test both the rocket and the Orion crew capsule where astronauts will sit on future missions. In particular, NASA wants to make sure that the heat shield on Orion can survive a fiery entry through Earth’s atmosphere at 25,000 miles per hour, the speed of a spacecraft returning from the moon.
Monday’s scrubbed launch added another delay to the moon program, named Artemis, which has already cost more than $40 billion and is years behind schedule. The program, including the giant rocket, has nonetheless received steady support from Congress and NASA officials.
The issue that halted the launch on Monday was a liquid hydrogen line that did not adequately chill one of the rocket’s four core-stage engines, part of the preparations needed before ignition. Otherwise, sudden shrinkage from the temperature shock of supercold propellants crack the metal engine parts.
Troubleshooting efforts proved unsuccessful within the limited time, and at about 8:40 a.m. Eastern, Charlie Blackwell-Thompson, the launch director, decided that it was time to call it off and try again another day. Even if they had resolved the technical issues, weather conditions may have prevented a launch.
“This is a brand-new rocket,” Bill Nelson, the NASA administrator, said during a news conference in the afternoon. “It’s not going to fly until it’s ready.”
If the launch cannot occur during the Labor Day weekend, the rocket will have to be rolled back to the giant Vehicle Assembly Building — essentially a garage for rockets. A trip there would most likely mean a delay of a month or more.
NASA officials said it was important to prudently tackle each problem as it arose and not to rush decisions that might lead to catastrophic failures.
“We are going to give the team time to rest, first of all, and then come back fresh tomorrow and reassess what we learned today and then develop a series of options,” said Mike Sarafin, the Artemis mission manager. “It’s too early to say what the options are.”
Had it lifted off, the flight would have capped a strong summer for NASA, which lit up imaginations all over the world when it released the first views of the cosmos captured by the powerful James Webb Space Telescope at the start of July.
Instead, NASA’s engineers, V.I.P. spectators and the public at large were disappointed, but many were sympathetic.
That included Ms. Harris, who had been scheduled to deliver a speech after an Artemis I launch. Instead, she spoke to reporters on Monday after NASA scrubbed the flight.
“Innovation requires this kind of moment where you test out something that’s never been done and then you regroup,” she said. “And you figure out what the next step will be to get to the ultimate goal, which for us is going to the moon and showing how humans can live and work on the moon.”
Ms. Calibeo, who studied aerospace engineering in college, said she was hoping the launch would still happen in the coming days. “There are so many people here and the excitement was crazy and definitely sad,” she said, “and hopefully I get to stick around.”
Kendal Van Dyke, 46, a senior program manager at Microsoft who lives in Orlando, and members of his family were set to watch the launch from the NASA Causeway. While disappointed, he emphasized that scrubbed launches were a standard risk in spaceflight.
“It’s not about wowing people. It’s about getting billions of dollars’ worth of hardware into space safely,” Mr. Van Dyke said. “Sometimes it works out and sometimes it doesn’t but that’s OK. We got a good experience and got to spend some time together.”
Six of his seven siblings traveled from around the region to watch the launch together and commemorate their father, who died in November and worked as a contractor on the Apollo program installing A.V. equipment to monitor astronauts on the launchpad. Several of his siblings now also work in the space industry.
“We thought it would be a great way to celebrate his passing and the accomplishments of the family” Mr. Van Dyke said.
It is not uncommon for technical problems to crop up during debut launch attempts. In 1981, the first space shuttle, Columbia, was on the launchpad with two astronauts strapped in for the first launch to orbit, but the countdown was halted by a computer glitch. Columbia successfully launched on the second try two days later.
For the Space Launch System rocket, the countdown started Saturday. Despite several lightning strikes on the launch site on Saturday afternoon, the countdown continued smoothly for the most part through the weekend. Then early Monday morning, the threat of nearby thunderstorms caused a 45-minute delay before liquid oxygen and liquid hydrogen could begin flowing into the rocket’s propellant tanks.
Another problem cropped up when a leak was detected in a hydrogen fuel line that attaches to the bottom of the rocket. That was a recurrence of a problem that occurred during a practice countdown in April.
Engineers were able to fix that problem, and the filling of the hydrogen tank resumed.
The engine issue that arose later in the countdown also involved hydrogen but in a different part of the rocket. In the last part of the launch countdown, some liquid hydrogen and liquid oxygen is diverted to flow around the four engines to cool them in preparation for ignition.
Three of the four engines were fine but, in the fourth, a hydrogen line did not appear to open properly, and one of the engines was not as cold as the others.
This was the first test of the engine chill-down, which usually occurs 4 minutes 40 seconds before launch. Dress rehearsals of countdown procedures earlier this year were designed to catch such issues but were cut short by technical problems. As a result, the engine chill-down was not tested. But mission managers believed the rocket had passed the critical test objectives, and they moved ahead with preparations for launch.
For Monday’s countdown, a chill-down test was added at an earlier point to allow troubleshooting in case a problem showed up. Mission managers recognized the risk.
“That is something that we’re going to demonstrate, end to end, for the first time on the day of launch,” Mr. Sarafin said last week after the mission team decided to go ahead with the launch attempt. “And if we do not successfully demonstrate that, we are not going to launch that day.”
Mr. Sarafin turned out to be correct.
Kenneth Chang reported from Kennedy Space Center, and Christine Chung from New York. Zolan Kanno-Youngs contributed reporting from Washington.