There's been a lot of rain at Kennedy these last few days. Everyone is watching the weather. Can they find a favourable window around launch time (16:33 local / 21:33 BST)? Remember, this is an instantaneous launch opportunity: you either go on the button or you don't. The reason for that is because the capsule has to catch the space station in the sky. If you hang around on the pad, this quickly becomes impossible. It all comes down to timing.

Image source, NASA

Elon Musk (L), the CEO at SpaceX, and Jim Bridenstine (R), the Nasa Administrator, have entered the suit-up room for one last face-to-face conversation with the astronauts.

Image source, NASA

Doug Hurley and Bob Behnken are in their pressure suits with the technicians now checking these complex garments are working properly. The suits have essentially two layers: an inner bladder and an outer skin. They control pressure and temperature and when the visor is shut, they control the oxygen environment, too. Notice how the technicians are all wearing masks. This is all part of the Covid protection procedures.

Image source, NASA

It's a big day for SpaceX, obviously. Ever since its formation in 2002, it's been building to this moment: the transport of humans into space. Controllers report no issues of concern right at this minute.

Astronauts Doug Hurley and Bob Behnken should have had their weather briefing by now. Time to suit up. The SpaceX "close-out" team will be heading down to the pad ready to receive them.

Image source, NASA

The head of Nasa says launch activities will proceed for the time being, even if the weather is somewhat dynamic.

David Shukman
Science editor, BBC News

With huge rocket components stretched out under a vast roof, the SpaceX factory in California had the feel of a futuristic film set back when I visited in 2011.

In fact, it had been used to shoot the movie Iron Man and a giant statue of the character stood over the production line.

Beside it was a prototype of the Crew Dragon spaceship due to launch later today, and I was invited to climb inside.

Bigger than the Apollo capsules that took men to the Moon, I reached it via a flight of steps before reversing in through a hatch.

It felt cool and dark inside – the only light came through the doorway and a porthole – and I remember wondering how it could accommodate the seven people it was designed for.

My questions to the founder of SpaceX, Elon Musk, were all about how safe the astronauts would be – to which he answered that no private company could tolerate accidents.

What he wanted to talk about his vision for life beyond Earth. It was when I first heard him say that humanity has a destiny as a "multi-planetary species".

If the weather allows, today’s task is more modest: getting two astronauts off the ground and back again without mishap.

Jonathan Amos
Science correspondent, BBC News

A word about what Hurley and Behnken will be doing when they get to space.

This mission is all about putting the Crew Dragon capsule through its paces. The design flew a year ago, but only with a "dummy" inside. SpaceX and Nasa need to understand how real people will engage with working and living inside the vessel.

Hurley and Behnken have about 19 hours before arriving at the 420km-high space station. In that time they will check out all the systems, including doing some manual flying.

Dragon is a totally automated ship. That's to say, you could just sit there and its computers would take you to your destination. But the pilots need to practise for all eventualities; and they want to know that the simulators on the ground reflect the true responses on the capsule in flight.

For aviators like Hurley and Behnken, the strangest thing is the absence of a control stick. Dragon is manoeuvred by entering commands on a touchscreen. The astronauts concede the different approach took some getting use to.

Image source, SPACEX

It was a big question ahead of the flight - what would the crew have to eat before leaving Earth? No-one seemed to have an answer earlier in the week. Now we do.

Supper in the capsule this evening will be soldier food: Meal, Ready-to-Eat (MRE). Perhaps not so appetising.

Nasa astronauts Doug Hurley (L) and Bob Behnken (R) have arrived at Kennedy's Operations and Checkout Building. They'll go through final briefings before getting into their space suits. It's then about a 6km drive down to the launch pad at Complex 39A. The schedule calls for them to get to the rocket and capsule about three hours prior to launch.

Jonathan Amos
Science correspondent, BBC News

Everyone is keeping their eye on the weather. SpaceX and Nasa have strict criteria that have to be met before they will proceed with a launch. The winds, the cloud cover, precipitation and lightning – all these factors must be watched very closely. And it’s not just the conditions at ground level on the launch pad that matter. Controllers are interested in what the weather is doing at different altitudes as the rocket climbs into the sky and heads out across the Atlantic Ocean. Going into Wednesday, the forecasters were saying there was a 50:50 chance of favourable weather for the launch. But crucially this forecast was only for the Kennedy launch site itself.

SpaceX and Nasa also need to listen to what the meteorologists are telling them about the conditions along the flight line in the event that the Dragon has to abort its ascent. If something goes wrong with its rocket, the capsule will need to splashdown somewhere along a swathe of ocean that stretches all the way up the eastern seaboard of the US and reaches across to the British Isles. SpaceX has designated certain zones where the crew would ditch, and each one has to have benign wave and wind conditions. If any of these zones step outside the permitted criteria, the launch will not proceed… even if it’s a beautiful day back in Florida.

And just to add to the complication, a tropical storm has formed off South Carolina.

Image source, NOAA

Image source, SpaceX

Why is SpaceX - a private company - launching astronauts from Nasa? Who is Elon Musk? Why is the launch so important?

There hasn't been a launch quite like this up until now, so naturally, it has raised many questions. The Crew Dragon is the first private space vehicle to carry astronauts to the International Space Station.

It's also the first time humans have launched from Kennedy Space Center in Florida since the retirement of the space shuttle in 2011. But a lot has changed in that time: the Crew Dragon is a very different vehicle, not only in its overall design - which recalls the capsules of the Apollo era - but also the technology inside.

The astronauts will fly the vehicle using touchscreen technology, rather than by using buttons, switches and a control stick - as they had to do for the space shuttle.

We've answered some key questions about the mission so that you can brush up on your knowledge ahead of today's historic launch.

Paul Rincon
Science editor, BBC News website

Image source, Anne-Katrin Purkiss

British astronauts Helen Sharman and Tim Peake have been sharing their thoughts on the SpaceX launch.

Sharman, who became the first Briton in space when she flew to the Russian Mir space station in 1991, said the mission would pave the way for "access to space for everybody".

"People around the world will fly because their space agencies are paying SpaceX to do it, but that does release other seats," she told the BBC News Channel.

Peake, who was the first British astronaut to fly to the International Space Station, called the mission "a game changer".

"We want to go back to the Moon... we ultimately want to get to Mars. In order to do that we need the help of commercial companies to take over low-Earth orbit and to be a partnership to provide things like launch ships, supply craft."

On what future criteria might need to be met by paying passengers looking to fly on vehicles like the Crew Dragon, Sharman explained: "People will have to be moderately healthy... like flying on airlines, it's going to become more commonplace. So long as nothing goes wrong, you won't have to be hugely physically fit."

Peake added: "What we're doing is opening up spaceflight to more and more people. There will have to be an element of medical selection. But I think this is going to become... more and more normal in the next 10 to 15 years."

Commenting on what she found interesting about the upcoming launch, Ms Sharman said: "This is going to have a splashdown - I think it's the re-entry that's going to be an interesting prospect... in particular, rescuing people in the water.

"What will be interesting is how do we do that with astronauts who have spent many months in space."

Indeed, astronauts coming back to Earth after a long spell on the ISS often find it difficult to move around and walk because of the muscle wastage they experience in orbit.

Image source, AFP

Image source, NASA

The "Starman" spacesuits that will be worn by Doug Hurley and Bob Behnken today have been getting rather a lot of attention online.

In contrast to the bulky suits of the past, with their "bubble" helmets, these are elegant, slimline and wouldn't look out of place in a sci-fi film.

That's probably no coincidence, because their look was devised by Hollywood costume designer Jose Fernandez who has worked on films such as Captain America: Civil War, Batman v Superman: Dawn of Justice and Tron: Legacy.

So how do they compare with other attire worn in space? Take a look for yourself in our gallery.

Image source, SpaceX

Some time after 13:00 EDT (17:00 GMT / 18:00 BST) we expect to see the astronauts walk out and get into cars for transportation to the launch pad.

The timing for today's launch has to be precise. The Falcon 9 rocket must leave the ground at 16:33 EDT (20:33 GMT / 21:33 BST), or the astronauts won't be able to catch the International Space Station (ISS) which passes overhead at 27,000km/h (17,000mph).

The rocket carrying Bob Behnken and Doug Hurley takes more than eight minutes to get to orbit. Once the Crew Dragon has separated, it makes the rest of the journey to the ISS using its own thrusters.

But one section of the rocket will touch down softly on a ship using engine power so that it can be re-used on a future flight. This is a SpaceX speciality that sets its Falcon apart from all other orbital rockets in use today.

Read the article here.

Skywatchers in the UK should be able to see the International Space Station and the Crew Dragon capsule pass over the United Kingdom later.

The space station will pass over first, at around the time of launch. Then, the Crew Dragon, carrying Doug Hurley and Bob Behnken, will fly past around 25 minutes after lift-off (roughly 21:58 BST).

It's a unique and historic opportunity to see them both before the rendezvous with the ISS some 24 hours later.

The astronauts have been making the most of the last hours before they launch into space. Nasa's administrator Jim Bridenstine checked in with crew members Bob Behnken and Doug Hurley on Tuesday. The crew members were quarantined behind glass - procedures are even more stringent than usual because of the Covid-19 crisis.

And Karen Nyberg, the Nasa astronaut who is married to Doug Hurley, tweeted a picture of herself, Hurley and their 10-year-old son Jack taken from the service structure on the launch pad at Kennedy Space Center.

Image source, Nasa

Nasa is sending two of its most experienced astronauts on the Crew Dragon: Bob Behnken and Doug Hurley. For the last two years, the men have been working with SpaceX to get the spacecraft ready for its first flight with humans.

Both astronauts have two space shuttle flights under their belt, and Hurley piloted the last ever shuttle mission in 2011. He'll now act as spacecraft commander on the first crewed flight from US soil since that historic launch nine years ago.

And when they're strapped into their seats in the capsule, waiting to be launched, their spouses will know exactly what's going through their heads. That's because they are astronauts too.

Read the profile of Bob Behnken and Doug Hurley.

Welcome to the BBC's live coverage of the first space launch with astronauts from US soil in nine years. At 16:33 EDT (20:33 GMT; 21:33 BST). Nasa crew members Doug Hurley and Bob Behnken will lift off from Florida's Kennedy Space Center atop a Falcon 9 rocket.

They'll be sitting inside the revolutionary Crew Dragon spacecraft, which - like the Falcon 9 - is built by Elon Musk's company SpaceX. And they'll lift off from Pad 39A, where the Apollo 11 mission started its journey back in 1969. The mission will also make history, becoming the first commercial launch to carry humans to the International Space Station.

It also marks a a shift to the commercialisation of human space transportation, with companies selling "taxi" rides to government and anyone else who wants to purchase a ride. So strap into your seats and get ready for an exciting day, as the BBC brings you full coverage of the Demo-2 mission.

Image source, SpaceX