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The US space agency has had to postpone the launch of its new Artemis I Moon rocket for the second time in a week. 6park.com
Controllers were unable to stop a hydrogen leak on the vehicle, almost from the start of Saturday's countdown procedure. 6park.com
Nasa now has another opportunity to launch the rocket on Monday or Tuesday. 6park.com
After that the vehicle will have to return to its assembly building for inspection and maintenance, which will mean further delays. 6park.com
The Space Launch System is the most powerful rocket ever developed by Nasa, and is designed to send astronauts and their equipment back to the lunar surface after an absence of 50 years. 6park.com
Nasa calls off new Moon rocket launchA photographic mission to show us the MoonArtemis: To the Moon and beyond 6park.com
Much of the SLS's enormous thrust comes from burning almost three million litres of super-cold liquid hydrogen and oxygen in four big engines on the vehicle's underside. 6park.com
But when controllers sent the early morning command to fill the rocket's hydrogen tank, an alarm went off, indicating there was a leak. 6park.com
The problem was traced to the connection where the hydrogen was being pumped into the vehicle. 6park.com
Controllers tried a number of fixes, including allowing the hardware to warm up for short periods, hoping this might reset the seal. But without success. 6park.com
The Artemis I mission is an uncrewed demonstration, but Nasa Administrator Bill Nelson said the rocket's future role in human spaceflight meant extreme care was still required in its operation. 6park.com
"We will go when it's ready," he stressed. "We don't go until then, and we make sure it's right before we put humans up on the top of it." 6park.com
It's possible Nasa could try again in the next few days. But there are battery systems on this rocket that will soon need inspection. And if the vehicle has to be rolled back to the engineering building for further work, it could be mid-October before we see it again on the launch pad. 6park.com
Saturday's attempt to despatch the Space Launch System (SLS) rocket had been scheduled for the start of a two-hour window beginning at 14:17 local time (19:17 BST; 18:17 GMT). 6park.com
The 100m-tall vehicle's objective was to hurl a human-rated capsule in the direction of the Moon, something that hasn't happened since Project Apollo ended in 1972. 6park.com


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How we get humans back to the Moon 6park.com
Nasa first tried to launch SLS on Monday, but that bid was ultimately scrubbed because controllers couldn't be sure the four big engines under the rocket's core-stage were properly prepared for flight. 6park.com
When the SLS does get away, it is sure to be a spectacular sight. 6park.com
"It's gonna be 'shuttle on steroids'," said Doug Hurley, who was the pilot on the very last shuttle mission in 2011. 6park.com
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Media caption,
Former astronaut Doug Hurley: "We're building on safe, capable designs" 6park.com
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The former astronaut now works for Northrop Grumman who make the big white solid boosters on the sides of the SLS. 6park.com
"What I always thought was the coolest thing about shuttle launches was you saw it lift off and it was well clear of the tower before you heard anything, and then it was even a little longer before you felt it," he explained. 6park.com
"Thrust to weight-wise, SLS is pretty close to what shuttle was. Apollo's Saturn V rocket was drastically different. I never saw it in person but it lumbered clear of the pad. For shuttle, it seemed like it was clear in an instant, almost as soon as the boosters were lit. SLS should be the same," he told BBC News. 6park.com


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The first powered phase of the SLS's ascent will last just over eight minutes. 6park.com
This will put the upper-stage of the rocket, with the Orion capsule still attached, into a highly elliptical orbit that would see the two of them come crashing back to Earth without any further effort. 6park.com
So, the upper-stage will have to raise and circularise the orbit before then boosting Orion in the direction of the Moon. 6park.com
Confirmation that the capsule is on its own, on track and speeding through space at 30,000 km/h (19,000mph) should come two hours and five minutes after launch. 6park.com
The planned mission length is just under 38 days, much longer than the 21 days that capsule manufacturer Lockheed Martin says is the maximum time astronauts should spend in the spacecraft. 6park.com

IMAGE SOURCE,NASA
Image caption,
Artwork: The upper-stage of the rocket will put the Orion capsule on a path to the Moon