As the US space agency NASA on Saturday resumed its second effort to get its giant 30-story rocket off the ground and send its uncrewed test capsule toward the Moon, engineers detected a fuel leak that could lead to a new delay.
With millions around the globe and hundreds of thousands on nearby beaches waiting for the historic launch of the massive Space Launch System, the leak near the base of the rocket was found as ultra-cold liquid hydrogen was being pumped in.
NASA said engineers would "stop flowing liquid hydrogen to the tank, close the valve used to fill and drain it," then try to reseal it.
The space agency offered no information on the likelihood of a new delay in the Artemis program -- which aims eventually to return a human crew to the Moon -- following Monday's aborted effort.
Saturday's launch is still scheduled for 2:17 pm (1817 GMT) from Kennedy Space Center in Florida. It can be delayed by up to two hours if the extra time is needed.
"Our team is ready," Jeremy Parsons, deputy manager of exploration ground systems at Kennedy Space Center, had said Friday.
"They are getting better with every attempt and actually performed superbly during launch countdown number one ... I think if the conditions with weather and the hardware align, we'll absolutely go."
Launch attracts 400,000 spectators
Though the area around the launch site will be closed to the public, an estimated 400,000 people were gathering on beaches nearby to see -- and hear -- the most powerful vehicle that NASA has ever launched climb into space.
The initial launch attempt was halted after engineers detected a fuel leak and a sensor showed that one of the rocket's four main engines was too hot.
Both issues have since been resolved, the launch team said.
Shortly before 06:00 am (1000 GMT) Saturday, launch director Charlie Blackwell-Thompson had given the go-ahead to start filling the rocket's tanks with fuel.
About three million liters of ultra-cold liquid hydrogen and oxygen was expected to be pumped into the spacecraft.
The weather appeared to be cooperating: the US Space Force predicts a 60 percent chance of favorable weather at the scheduled liftoff time, growing to 80 percent later in the launch window.
If something requires NASA to stand down again on Saturday, there are backup opportunities on Monday or Tuesday. After that, the next launch window will not be until 19 September, due to the Moon's position.
The purpose of the Artemis 1 mission is to verify that the Orion capsule 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.
