Image source, Reuters

We're putting our live coverage on pause for now, while Nasa engineers try to resolve the issues that have cancelled today's launch.

But we'll be back as soon as they do, so join us then to follow the launch of Nasa's biggest ever rocket.

In the meantime, here's a round-up of what's been happening today:

• Nasa was due to launch its most powerful ever rocket - the Space Launch System - from Florida's Kennedy Space Center
• This would have been the first test flight in the Artemis programme - to take humans back to the Moon for he first time in 50 years
• But the launch has been hampered by technical glitches - first a crack was found, then engineers were trying to stabilise the temperature of one of the big engines under the rocket
• The launch was finally called off when Nasa announced an "engine bleed" could not be fixed in time
• Nasa can try again on Friday, or on 5 September

We'll have our engines fired up then to bring you the latest updates as they happen.

Image source, Getty Images

Hundreds of thousands of people travelled to Florida with the hope of seeing the launch of Nasa's new rocket.

But, as you've been hearing, it wasn't to be.

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

Rebecca Morelle
Reporting from the Kennedy Space Center

Image source, Reuters

Everyone at the Kennedy Space Center was waiting - and willing - this rocket to go up.

So there is a real sense of disappointment here, but not of surprise.

You have to remember that this is a new rocket - it's never been put through it's paces before - so a scrub was always possible.

But if the rocket isn't working perfectly - it can't fly - and everyone here accepts it.

There's no doubt they'll be trying again - now we're just waiting to find out when.

"You don't want to launch a candle until it's ready to go", says Nasa administrator Bill Nelson following the cancelled launch of Amtemis I.

Speaking on Nasa's live feed he said: “We don’t launch until it’s right”.

The rocket is a "very complicated machine" he said, adding that Nasa was testing the aircraft in "a way you would never do with the human crew on board.

"This is just part of the space business and particularly a test flight", he said.

Image source, Maxar Technologies

There's a lot of technical language to digest here - especially when it comes to the Artemis I rocket and all its parts.

If you're getting confused - or just want some extra detail and a visual - we've created a detailed animation of the Space Launch System (SLS) here.

Controllers ran out of time in the two-hour launch window to solve an engine bleed issue, Nasa says.

As we've reported, the launch of Armestis I was cancelled today, known as a scrub.

Nasa says launch controllers were continuing to evaluate why a bleed test was not successful and ran out of time.

The test was being conducted to get the RS-25 engines on the bottom of the core stage to the proper temperature range for lift-off.

Nasa says engineers will continue to gather additional data.

The cancelling of today's mission "isn't unexpected", says Dr Becky Smethurst, an astrophysicist at the University of Oxford, who points out that there were snags during dress rehearsals earlier in the year.

"It literally is rocket science," she tells the BBC.

Dr Smethurst commends the engineers involved, saying she imagines the atmosphere on the ground was "insane" as Nasa personnel rushed around trying to find a last-minute fix for an engine issue.

She says she's "confident" that a couple more days will be enough to fix any snags ahead of a second launch attempt on Friday.

Jonathan Amos
Reporting from Kennedy Space Center

Image source, Reuters

Space exploration is not cheap, especially when it involves hardware that must keep humans safe.

The Space Launch System and Orion have been in development for over a decade and have cost, in each case, more than $20bn (£17.1bn) to get to this point.

But what about the ongoing costs? A recent assessment from the Office of Inspector General (OIG), which audits Nasa programmes, found that the first four SLS missions would each cost more than $4bn (£3.4bn) to execute - a sum of money the OIG described as "unsustainable".

Nasa counters that it is changing the way it contracts industry and this will bring down future production costs significantly.

Hundreds of thousands of people travelled to Florida with the hope of seeing the launch of Artermis One on Monday.

However, as we've reported, the rocket launch has been cancelled - leaving crowds likely already considering their journey home.

There have been calls for motorists to "take heed" and watch out for people heading back.

Jonathan Amos
Science correspondent, BBC News

Image source, NASA

The American space agency has called off the launch of its big new Moon rocket - the Space Launch System.

Controllers struggled to get an engine on the 100m-tall vehicle cooled down to its correct operating temperature.

They’d previously worried about what appeared to be a crack high up on the rocket but eventually determined it was merely frost build-up.

The SLS is the biggest rocket ever developed by Nasa. It will be used to send astronauts back to the Moon.

The maiden flight is just a demonstration with no-one aboard, but ever more complex missions are planned for the future that will see people live on the lunar surface for weeks at a time.

Nasa has the option to try again on Friday, if the engine issue can be resolved easily.

But it’s possible the rocket may have to be rolled back to the assembly building at the Kennedy Space Center for more extensive work. This could see the flight slip further into September.

Launch director Charlie Blackwell-Thompson cancelled the planned launch of Artemis I as a result of an engine bleed issue, Nasa says.

The Space Launch System (SLS) rocket is said to be in a "stable configuration", and engineers will now work on gathering data to see what went wrong.

The rocket was due to be launched a short time ago, at 08:33 Florida time (13:33 BST).

The next available opportunity for a launch is 2 September, Derrol Nail of Nasa says, although he adds this is dependent on the fix for the engine bleed issue.

Nasa have just announced the launch director "has called a scrub for the day".

An engine bleed that "couldn't be remedied" is the cause.

Rebecca Morelle
Reporting from Kennedy Space Center

The countdown clock is still on hold here at Kennedy.

We've just heard that the team is working on a troubleshooting plan - one of the four huge engines that they need to launch the rocket can't cool down enough.

The flight director wants to look at the data and computer modelling to see what happens next.

The problem is that they're facing some major time constraints. The launch window is set to open at 13:33BST - and they only have two hours to get this rocket up.

Even if the engine issue is resolved, it's not clear if the team can catch up and get this rocket off the ground in time.

Rebecca Morelle
Reporting from Kennedy Space Center

Image source, .

The Space Launch System has been called the "mega Moon rocket" – for good reason.

Not only is it a colossal 98m high, it’s also the most powerful rocket Nasa’s ever built.

Standing on the launchpad, 90% of its weight is fuel: vast amounts are needed to get this monster off the ground.

So, it uses two enormous rocket boosters, as well four huge engines, to do the heavy lifting.

It needs all this power to escape the gravity of the Earth, and then push a spacecraft - called Orion - towards the Moon.

Orion is located near the top of the rocket, and it’s where the astronauts will eventually sit in future missions.

The spacecraft has an epic journey – it will fly more than a million miles – as it travels around the Moon and then returns to Earth with a splashdown in the Pacific.

Image source, .

Image source, Nasa

We've just heard more from Nasa while the countdown clock continues to be held at 40 minutes.

The hold was supposed to last for just 10 minutes, but has now gone on for around half an hour.

"We are standing by," says Derrol Nail of Nasa Communications on the space agency's video feed.

Nail says an engine "didn't get the high-accuracy temperature we were looking for".

He describes the issue as "particularly tricky", adding that this was something Nasa crews had unable to test beforehand.

It's a "constraint to launch", Nail adds.

Jonathan Amos
Reporting from the Kennedy Space Center

Image source, Getty Images

"I just have to say pretty bluntly here: we've been there before." That's what US President Barack Obama said when he cancelled the pre-Artemis project to get back to the Moon.

So why are we going back? Well, it's unfinished business, scientifically.

The Moon is where you go to find out things about the geological history of Earth. Our planet has erased much of its past, weathering and recycling its rocks.

The Moon preserves the conditions that existed early in the Solar System, billions of years ago. We go to the Moon to learn about us.

But we also go there to learn how to go to Mars.

The Moon is not far away. If you get into trouble, you can come back quickly. Getting to and from Mars is much more difficult, and if you choose to visit the Red Planet, you'd better be prepared.

A Nasa astronaut has been speaking to the BBC, and said the kind of technical issues currently being experienced are "very common".

"Especially for the first flight of a brand new spacecraft," Stan Love says, adding this is the first time the rocket's been brought close to take off.

He says this is a test flight, meaning there are "many opportunities for new things to crop up" but adds "I really hope we're ready".

On the planned journey itself, Love says it'll be a six-week flight where engineers and astronauts' main aim is to "make sure every part of that spacecraft works [before we] bring it back to Earth".

Providing everything goes to plan, he says a crew will be named upon Artemis' return and training will begin soon after - bringing us one step closer to humans reaching the Moon again.

While the Artemis I was originally scheduled to launch at 08:33 local time (13:33 BST) from the Kennedy Space Center in Florida, it can be pushed back as far as 10:33 local time (15:33 BST) if necessary due to weather or technical issues.

If take-off isn't possible during that time frame, Nasa has two back-up dates ready: 2 September and 5 September.

These launch windows are largely driven by the rocket's Flight Termination System (FTS), which has a battery certified for 20 days, beginning with its installation on the SLS on 18 August.

If the take-of doesn't take place by then, the FTS will have to be removed and replaced.

Nasa officials, however, have expressed confidence that the take-off will take place on one of the three scheduled dates.

Image source, Getty Images

Jonathan Amos
Reporting from Kennedy Space Center

The crack issue is now not considered serious. Engineers believe what they've been looking at is simply ice formation on the exterior of the core stage of the rocket. But engineers continue to work other issues that may yet lead to a scrub for the day.

The key outstanding issue right now appears to be getting one of the big engines under the rocket down to its correct operating temperature.

Image source, NASA

Image source, NASA

More drama as the countdown clock for the launch has been put on hold, stopping at 40 minutes.

Nasa says it is being held for 10 minutes while the hydrogen team brief the flight director.

Engineers are unable to get an even engine temperature across all four engines, the BBC understands.

Work is being done to try and resolve the issue and the engineers have asked the launch director for more time.