If all goes well during the three-week flight, the rocket will propel an empty crew capsule into a wide orbit around the moon, before the capsule returns to Earth with a splashdown in the Pacific in December.

After years of delays and billions in cost overruns, the Space Launch System (SLS) rocket thundered skyward, rising from Kennedy Space Center on 4 million kg of thrust and hitting 160 km/h within seconds.

“I’m telling you, we’ve never seen such a tail of flame,” said NASA Administrator Bill Nelson, who followed the launch with a group of astronauts.

“There were a bunch there that would like to be on that rocket and I have to say, for what we saw tonight, it’s an A-plus,” he said.

The Orion capsule was perched on top the rocket and, less than two hours into the flight, busted out of Earth’s orbit toward the moon.

“For the Artemis generation, this is for you,” launch director Charlie Blackwell-Thompson said shortly before liftoff, referring to people who were not alive for the Apollo program, which ended 50 years ago.

She later told her team: “You have earned your place in history.”

The launch marked the start of NASA’s Artemis lunar exploration program, named after Apollo’s mythological twin sister.

The space agency is aiming to send four astronauts around the moon on the next flight, in 2024, and land humans there as early as 2025.

The launch follows nearly three months of vexing fuel leaks that kept the rocket bouncing between its hangar and the pad.

A series of hydrogen fuel leaks plagued the summertime launch attempts as well as countdown tests.

A fresh leak erupted at a new location during Tuesday night’s fueling, but an emergency team managed to tighten the faulty valve on the pad.

Then a US Space Force radar station went down, resulting in another scramble, this time to replace an ethernet switch.

The 98-meter SLS is the most powerful rocket ever built by NASA, with more thrust than either the space shuttle or the mighty Saturn V that carried men to the moon.

Orion should reach the moon by Monday, more than 370,000 km from Earth. After coming within 130 km of the moon, the capsule will enter a far-flung orbit stretching about 64,000 km beyond.

Artemis Mission Manager Mike Sarafin said the rocket is mostly performing as it should.

He said there were a few minor issues, which he called “funnies,” but Sarafin and other officials emphasized that all systems were “performing.”

Orion Program Manager Howard Hu said NASA would continue to test the engines and Artemis’ other functions, especially in the conditions of space.

At a press conference following the launch, Nelson said: “This is just the test flight, and we are stressing it and testing it in a way that we would not do with a human crew on it.

“But that’s the purpose, to make it as safe as possible, as reliable as possible, for when our astronauts crawl on board and go back to the moon.”

The $4.1 billion (€3.9 billion) test flight is set to last 25 days, roughly the same as when crews will be aboard.

The space agency intends to push the spacecraft to its limits and uncover any problems before astronauts strap in.

The mannequins — NASA calls them moonequins — are fitted with sensors to measure such things as vibration, acceleration and cosmic radiation.

“There’s a fair amount of risk with this particular initial flight test,” said Sarafin.

The rocket was supposed to have made its dry run by 2017. Government watchdogs estimate NASA will have spent $93 billion (€90 billion) on the project by 2025.

Ultimately, NASA hopes to establish a base on the moon and send astronauts to Mars by the late 2030s or early 2040s.