NASA scrubbed a test flight on Monday of its largest-ever rocket in a setback to the ambitious program to send humans back to the Moon and eventually to Mars.
"We don't launch until it's right," NASA administrator Bill Nelson said after an engine issue forced a cancellation of the launch from the Kennedy Space Center.
"This is a very complicated machine," Nelson said. "You don't want to light the candle until it's ready to go."
Alternative dates for the launch of the US space agency's uncrewed Artemis 1 mission are Friday and next Monday.
Blastoff had been planned for 8:33 am (1233 GMT) but was cancelled because of a temperature problem with one of the four RS-25 engines on the 322-foot (98-meter) Space Launch System (SLS) rocket.
NASA said a test to get one of the main engines to the proper temperature range for blastoff was not successful.
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Delays are "part of the space business," Nelson said, expressing confidence that NASA engineers will "get it fixed and then we'll fly."
Tens of thousands of people -- including US Vice President Kamala Harris -- had gathered near the Kennedy Space Center in Florida to watch the launch, which comes 50 years after Apollo 17 astronauts last set foot on the Moon.
"While we hoped to see the launch of Artemis 1 today, the attempt provided valuable data as we test the most powerful rocket in history," Harris tweeted. "Our commitment to the Artemis Program remains firm, and we will return to the Moon."
Stan Love, a veteran NASA astronaut, told reporters he was disappointed but "not really surprised."
"This is a brand new vehicle," Love said. "It has a million parts. All of them have to work perfectly."
The goal of the flight is to test the SLS and Orion crew capsule that sits on top. Mannequins equipped with sensors are standing in for astronauts.
Overnight operations to fill the orange-and-white rocket with more than three million liters of ultra-cold liquid hydrogen and oxygen were briefly delayed by a high risk of lightning.
Around 3:00 am, a potential leak was detected during the filling of the main stage with hydrogen, causing a pause. After tests, the flow resumed.
NASA engineers later detected the engine temperature problem and put a hold on the countdown before scrubbing the launch altogether.
The Orion capsule is to orbit the Moon to see if the vessel is safe for people in the near future. At some point, Artemis aims to put a woman and a person of color on the Moon for the first time.
During the 42-day trip, Orion will follow an elliptical course around the Moon, coming within 60 miles (100 kilometers) at its closest approach and 40,000 miles at its farthest -- the deepest into space by a craft designed to carry humans.
One of the mission's primary objectives is to test the capsule's heat shield, which at 16 feet in diameter is the largest ever built.
On its return to Earth's atmosphere, the heat shield will have to withstand speeds of 25,000 miles per hour and a temperature of 5,000 degrees Fahrenheit (2,760 degrees Celsius) -- roughly half as hot as the Sun.
Crewed mission to Mars
The dummies aboard the spacecraft will record acceleration, vibration and radiation levels.
The craft will also deploy small satellites to study the lunar surface.
NASA is expected to spend $93 billion between 2012 and 2025 on the Artemis program, which is already years behind schedule, at a cost of $4.1 billion per launch.
The next mission, Artemis 2, will take astronauts into orbit around the Moon without landing on its surface.
The crew of Artemis 3 is to land on the Moon in 2025 at the earliest.
And since humans have already visited the Moon, Artemis has its sights set on another lofty goal: a crewed mission to Mars.
The Artemis program aims to establish a lasting human presence on the Moon with an orbiting space station known as Gateway and a base on the surface.
Gateway would serve as a staging and refueling station for a voyage to Mars that would take a minimum of several months.