Nasa and SpaceX are about to launch two astronauts into orbit and make history.
It will be the first time a private company has put humans into space, and the first time that Nasa has sent its astronauts into orbit from US soil since the end of the Space Shuttle programme in 2011.
The launch begins at 4.33pm local time, or 9.33pm in the UK.
It can be viewed in the live stream below – and there may even be the opportunity to spot the launch from the ground.
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Here's Nasa's live feed of the launch:
The important thing about the weather isn't only the conditions in Florida. It also requires good weather across the Atlantic, because if the astronauts were have to abort their mission they would need to be sure the sea would be safe for them to drop into.
It's just a waiting game for now. We'll get a weather update in about two minutes and we should know.
But the tone on the official feed is getting more damp: they're talking more and more about the importance of abiding by the weather and the safety issues that not doing so would present.
We'll get a weather update in a few minutes, which will give an indication of whether conditions have cleared enough to allow the countdown to go ahead.
We're now less than 30 minutes away from launch. And the countdown is continuing, which suggests the weather hasn't entirely got in the way just yet...
Nasa TV notes that today has an instantaneous launch time. That means that there is no room for any kind of flexibility, or waiting for weather to pass. If they don't use the scheduled time, in 27 minutes, then they'll have to wait for the weekend.
Propellant loading has started. Liquid oxygen and RP1 – rocket grade kerosene – is being put into the rocket.
The launch escape system is now armed, SpaceX says. That would allow the capsule to shoot off of the rocket if something went wrong and the astronauts had to get free.
That means the propellant – the rocket fuel – that will actually carry them into space can start being loaded in.
The arm is now moving away from the dragon capsule, leaving the astronauts on their own up there. (They were just given a briefing on how they would escape if anything were to go wrong.)
Here's how that looks from the arm itself.
Here's Donald Trump on his tour of Kennedy Space Center earlier.
There might even be the chance to see the rocket from the ground too. Instructions on that are here:
How to watch SpaceX rocket launch in skies above the UK
Clear skies mean people will be able to catch the Falcon 9 rocket soar over the skies of the UK on WednesdayThe weather is still causing problems. While the earlier concerns about lightning have lifted, there are worried about the clouds.
So we're "no go" on that basis for now. But that might change as we progress towards launch.
Johnson Space Center in Houston – which of course doesn't have as much of a role to play here, given that it's SpaceX controlling the mission, but still has to ensure that everything is OK with the International Space Station and more – says that it is "go".
JSC and everyone else is still watching the weather to see if it will allow a launch today.
Everything's running very quickly. We're 20 minutes ahead of schedule.
We're about 75 minutes away from launch. That will happen at 4.33pm local time, or 9.33pm in the UK.
Jim Bridenstine, who has been Nasa administrator for nearly three years, described the joint effort with SpaceX to send astronauts into low-Earth orbit as a "monumental achievement".
Speaking ahead of the launch, he said: "We have had challenges. We have had setbacks. We have seen catastrophic losses of capsules and challenges with parachutes on the ground.
"But that's what is unique about SpaceX. SpaceX can do things that Nasa historically has not done."
He added: "We are ready to go. But if you had told me two years ago we would be right here I might have even been questioning it then. But this is a monumental achievement.
"It is a Herculean task by the SpaceX team, which we are very grateful for, and also by the Nasa team, that has been working hand-in-glove with them to get to this point.
90 minutes until the scheduled launch.
That weather is still looking very threatening. Here's a photo from nearby.
And here are radar pictures that tell a similar story.
Here's Air Force One – with Donald Trump on board – coming into land at the Shuttle Landing Facility and presumably getting an incredible view of the rocket and its launchpad.
The hatch has been closed! That happened earlier than expected.
Now the Pad Team are up to something. Not quite sure what it is! Commentators suggesting it's a leak check to ensure that everything is sealed as it should be.
Musk – looking emotional – reveals the words he said to the families as they said goodbye to the astronauts.
"We've done everything we can to make sure your dads come back OK," he says.
Elon Musk is speaking live, alongside Nasa administrator Jim Bridenstine. (They're inside because the weather is bad.)
"This is a dream come true," he says. "I think for me and for everyone at SpaceX. This is not something I ever thought would actually happen. When starting SpaceX in 2002, I really did not think this day would occur. I expected a 90% chance we'd fail to get to low earth orbit with a small rocket.
"[If you'd have told me about today] I would have said man what are you smoking?
"To say it's a dream come true – I didn't even dream it would come true."
Please allow a moment for the live blog to load.
