Jonathan Amos
Science correspondent, BBC News

Image source, Getty Images

The private rocket company SpaceX has sent two Nasa astronauts into orbit.

It's the first time since the retirement of the shuttles nine years ago that an American crew has made the journey from US territory.

Doug Hurley and Bob Behnken are not only trialling a new capsule system, they are also initiating a new business model for Nasa.

The agency will no longer own the vehicles it uses but merely purchase the "taxi" service offered by SpaceX.

Read the full story here.

Over the next 19 hours, Crew Dragon will execute a series of burns to keep raising its orbit until its in line with the ISS.

There will be five major burns before the final approach.

The first one is happening in a few minutes when Crew Dragon is at the highest point of its current orbit.

Temperatures will come down in the capsule soon too, and the astronauts will finally be able to remove parts of their space suits, which they've been in for hours already.

Nasa administrator Jim Bridenstine says at the moment of launch, he was praying.

"I've heard that rumble before but it's a whole different feeling when you've got your own team on that rocket," he says.

"They are our team, they are America's team, this is Launch America, this is everything America has to offer in its purest form."

The administrator says times are undoubtedly difficult right now, but he hopes the launch has helped inspire people.

"We've got the coronavirus pandemic, we've got other challenges as a country but I hope this moment in time is an opportunity for everybody to reflect on humanity and what we can do when we work together," he says.

"If this can inspire a young child to become the next Elon Musk, or the next Jeff Bezos, or the next Sir Richard Branson, then that's what this is all about."

Image source, Nasa

Nasa administrator Jim Bridenstine, who watched the launch with US President Donald Trump and Vice-President Mike Pence, says: "We have done it."

"This has been a long time coming. It's been nine years since we've launched American astronauts on American rockets from American soil and now it's done."

Bridenstine praised Trump's actions for the space programme, and was grateful for the budgets lawmakers in Congress, Republicans and Democrats, have allocated.

"I'm breathing a sigh of relief but I'm also gonna tell you I'm not gonna celebrate until Bob and Doug are home safely," he added.

Bridenstine is now heading to Houston, where he'll watch Dragon dock with the ISS.

Over 10 million people watched the launch live, according to Nasa.

Trump is now the third sitting president to watch a launch live from the Kennedy Space Center.

Behnken and Hurley are in orbit, feeling that zero gravity.

The pair now have a 19 hour trip to the ISS and they'll dock tomorrow morning.

But they've got work to do yet.

Remember, this is still a test flight - and the astronauts will be testing out the system and Dragon's controls throughout their trek to the ISS.

The Crew Dragon capsule is officially separated from the rocket and making its way towards the space station.

Image source, Nasa

The first stage of the rocket has just come down onto the ship, named Of Course I Still Love You.

It's the first return of a Falcon 9 rocket that carried humans into space.

The astronauts are travelling over Ireland at the moment, and it's just been minutes since lift-off in Florida.

Image source, Nasa

Image source, Nasa

It's a moment worth re-watching, to be sure.

If you missed anything the first time around, SpaceX has got you covered.

The first stage booster of the rocket is making its way back to land in a drone ship back on Earth. It will be beginning re-entry momentarily.

Crew Dragon is still using the second stage of the rocket to get into orbit.

Image source, Nasa

They're past the sound barrier now.

At this point, the astronauts are feeling 2.3Gs - that's 2.3 times the gravity we feel on earth.

And they're moving at over 1,500 miles per hour.

Everything is still looking good. Next up, we'll have main engine cut off and separation as they move into orbit.

It’s 15:22ET, and we have lift-off.

The Crew Dragon, atop the Falcon 9 rocket, has taken off from the same site used for the first moon landing in 1969. It’s the first private space vehicle to transport astronauts into orbit.

What happens next? They’ll reach their initial orbit in around nine minutes, and part of the rocket will return to earth to be reused.

Astronauts Douglas Hurley and Robert Behnken should reach the space station at around 10:30ET/15:30BST on Sunday.

Image source, nasa

Coming up on under nine minutes to go, the SpaceX team thanks the astronauts for their work, and says: "Enjoy those views of our beautiful planet."

"It is absolutely our honour to be a part of this huge effort to get the united states back in the launch business," says Hurley. "We'll talk to you from orbit."

Next up, they'll start the engine chill - which is required to make sure the liquid oxygen doesn't turn into gas as it moves through in preparation for ignition.

Image source, Nasa

If CrewDragon launches, the capsule will pass directly over the UK counties of Devon and Cornwall just over 20 minutes later at 20:43. You won't be able to see them from the ground; the sky will still be too light at that stage. But how about you go outside and wave to Doug and Bob anyway!

The first attempt to launch happened on Wednesday - so why did Nasa and SpaceX wait until today to try again?

There are only certain launch times that work with the space station's orbit - and the ISS is travelling overhead at over 17,000mph (27,000km/h).

That means that on different days, the time to reach the space station from the launch site varies, and the amount of fuel each trip needs also changes.

Today was the next best option from Wednesday, trajectory-wise, with an approximately 19-hour journey to the ISS after lift-off.

Once they reach orbit, about nine minutes into flight, Behnken and Hurley will test out the capsule's systems as they make their way to the ISS.

You can check out their trajectory in the SpaceX animation below.

We're still not sure how long the astronauts will remain aboard the ISS, but it will likely be between one to four months.

On the ground in Florida, we're still looking good weather-wise, with just under 20 minutes to go.

If anything happens after this point to delay, we'll have to wait until tomorrow to try once more.

So what's being loaded right now?

The fuel is known as RP-1 - that's rocket propellant, grade 1, refined kerosene.

The oxidiser, which provides the oxygen needed for burning, in this case is densified liquid oxygen - oxygen that's kept much colder than typical, which helps with the loading process.

Falcon 9 also uses another fluid that ignites in the presence of oxygen, and you might see a green flare from this a couple of minutes into flight.

The launch director has just given the go-ahead for propellant loading on the Falcon 9 rocket.

We're under 35 minutes to go for lift-off.

The launch escape system is designed to automatically fire if a dangerous condition is detected.

The capsule containing the astronauts will be pushed away from the rocket if that happens.

There also plans for return and recovery in the event that, for any reason, the Crew Dragon can't dock with the space station.

Nick Picon, the SpaceX in-flight abort mission manager, says it's an "absolutely critical" capability and has just been armed now on the launch pad as we continue to count down for lift-off.

Image source, Reuters

Today, two US astronauts will achieve a world first when they launch to the International Space Station aboard a spacecraft built by Elon Musk's SpaceX.

Doug Hurley and Bob Behnken are about to break a nine-year hiatus for Nasa, becoming the first astronauts to launch from US soil since the retirement of the space shuttle in 2011.

So who are they?

Check out our profiles of these two history-making astronauts here.