Nasa on Saturday called off its second attempt to launch its next-generation rocket to the Moon after engineers detected a fuel leak.
The next possible launch window is two weeks away after two attempts in a week were cancelled, both because of fuel leaks.
With millions around the globe and hundreds of thousands on nearby beaches waiting for the massive Space Launch System to blast off, a leak near the base of the rocket was found as ultra-cold liquid hydrogen was being pumped in.
“The launch director waived off today's Artemis I launch,” Nasa said. “Multiple troubleshooting efforts to address the area of the leak … did not fix the issue.”
Though the area around the site was closed to the public, an estimated 400,000 people had gathered near by to see the most powerful vehicle that Nasa has ever launched climb into space.
The next launch windows are September 19 to October 4 and then October 17 to 31, Nasa said.
The initial launch attempt on Monday was also halted after engineers detected a fuel leak and a sensor showed that one of the rocket's four main engines was too hot.
Early on Saturday, launch director Charlie Blackwell-Thompson gave the go-ahead to start filling the rocket's tanks with cryogenic fuel.
About three million litres of ultra-cold liquid hydrogen and oxygen were due to be pumped into the spacecraft, but the process soon hit problems.
The purpose of the Artemis 1 mission is to verify that the Orion capsule, which sits on top of the SLS rocket, is safe to carry astronauts in the future.
Mannequins equipped with sensors are standing in for astronauts on the mission and will record acceleration, vibration and radiation levels.
It will take several days for the spacecraft to reach the Moon, flying about 100 kilometres above it at its closest approach. The capsule will fire its engines to get to a distant retrograde orbit of 64,000km beyond the Moon, a record for a spacecraft rated to carry humans.
The mission is expected to last about six weeks and one of its main objectives is to test the capsule's heat shield, which at 16 feet in diameter is the largest built.
On its return to Earth's atmosphere, the heat shield will have to withstand speeds of 40,000 kilometres an hour and a temperature of 2,760°C, roughly half as hot as the Sun.
Artemis is named after the twin sister of the Greek god Apollo, after whom the first Moon missions were named.
Unlike the Apollo missions, which sent only white men to the Moon between 1969 and 1972, Artemis missions will send the first non-white person and the first woman to step foot on the lunar surface.
A US government audit estimates the Artemis programme's cost will grow to $93 billion by 2025, with its first four missions costing $4.1 billion per launch.
The next mission, Artemis 2, will take astronauts to the Moon without landing on its surface.
The crew of Artemis 3 will land on the Moon in 2025 at the earliest, with later missions envisaging a lunar space station and a sustainable presence on the lunar surface.
According to Nasa chief Bill Nelson, a crewed trip to Mars aboard Orion, which would last several years, could be attempted by the end of the 2030s.