The space shuttle was able to land on a runway like an aircraft, while the Russian Soyuz vehicle parachutes down to the Kazakh steppe. But the Crew Dragon will land on water - in the Atlantic Ocean.

Astronauts have not splashed down in a capsule since the days of the Apollo missions to the Moon in the 1960s and 70s.

Here's an illustration showing how the SpaceX vehicle will return from the International Space Station.

Image source, Nasa

The crew access arm is being moved away, and propellant loading will begin soon.

Weather for lift-off still looks good, and the clouds in the area - which were a concern - have also cleared up.

There's a 70% chance of good weather now, Nasa says. The conditions in the Atlantic Ocean - where the capsule would have to land in case of an emergency landing - are also looking good.

The weather appears to be improving, and we just heard the crew say launch is still go.

There was a weather condition the team was watching, but that has now cleared up, according to Nasa.

Propellant loading should begin shortly.

In addition to its astronauts, the Crew Dragon spacecraft is carrying two special - and perhaps unexpected - items.

One is a series of custom art pieces by a Los Angeles artist entitled "humankind". The indestructible paintings, meant to celebrate humanity, are made from gold, brass and aluminum. They also pay homage to the Saturn V rocket as well as the SpaceX Falcon 9 launcher.

A mosaic image of Earth, comprised of over 100,000 photos of 2020 graduates from around the world, is also travelling up with the astronauts.

Image source, Nasa

SpaceX released an online simulator earlier this month that lets anyone try their hand at piloting the Dragon 2 craft and docking with the space station.

The simulator uses the actual interface the astronauts will use.

As we countdown to lift-off here, you can have a go at steering the craft on the SpaceX site, here, external.

We've got less than an hour left until the planned lift-off.

The crew say they're go for launch - we're just waiting on the weather and should get an update shortly, at the 45 minute mark.

There have been some storms developing in the area, but Nasa says conditions have been improving, and they hopefully won't affect today's schedule.

Lauren Lyons, commentator on Nasa's live TV coverage, describes how she dreams of being able to land on Europa, the watery moon of Jupiter

"Scientists believe there is a liquid water ocean beneath the ice and is maybe habitable. And so, I'm super-excited to get up close and personal with maybe some space fish," she says.

Special guest host and former astronaut Leland Melvin gets asked where he would like to go. His dream destination is Mars, but only if he can bring his dogs.

"Having a Martian base that is dog-friendly - bring the pups along," he explains.

Here's a picture of Leland and his dogs from the space shuttle days.

Image source, NASA

The launch will be visible in the sky - if it goes ahead. Here's a handy guide for watching from the UK, from astronaut Tim Peake.

Heavy showers have been moving through Kennedy. The hope is they will all pass out over the coast come launch time. Fingers crossed.

Over the course of two launch attempts, Nasa TV commentators Lauren Lyons and Leland Melvin (who's a retired astronaut) have been bantering about which of the two iconic Nasa logo designs is better.

The round blue insignia is called the "meatball" and is Nasa's current official symbol. The "worm", so-called because of its stylised rendering of the Nasa acronym, came into use in 1975 and was retired in 1992. But recently, it's been making a comeback - including on the rocket launching today.

It turns out Leland is firmly in favour of the meatball and is not such a fan of the worm. Lauren is a big fan of the worm. But now, they've settled on a compromise: it's called the "wormball".

Image source, NASA

US President Donald Trump is returning to the Kennedy Space Center to watch the launch. He was at the spaceport on Wednesday for the aborted attempt. Here's a fun photo taken while he was on the way to Florida.

The astronauts are now reclined in their seats, waiting for the countdown to start.

Image source, NASA

Paul Rincon
Science editor, BBC News website

Last year, I interviewed Dr Michael Barratt, who graduated in the same astronaut class as Doug Hurley and Bob Behnken - as well as their respective spouses, Karen Nyberg and Megan McArthur.

Barratt, who qualified as a medical doctor before joining Nasa's astronaut corps, said the demands on today's astronauts have never been higher.

"The difference between the astronaut of today and the astronaut of 20 years ago in the US is quite vast. Twenty years ago there were short duration shuttle missions and they were very keenly focused on either a payload the shuttle was carrying, or visiting the International Space Station (ISS)."

Dr Barratt, who is also an honorary professor at the University of Exeter Medical School, UK, said astronauts of today had to be "uber-trained" in spacewalking, robotic arm operation and other systems. For the purposes of flying on the Soyuz, they also have to be functionally bilingual in English and Russian.

He described how being in space for long periods affects the human body. "You undergo a constellation of changes that include decrease in plasma volume,decrease in red blood cell mass, and neural vestibular reconditioning - I'll call it that - that makes you very good at navigating in three dimensions and not just walking on your feet," he said.

In addition to changes in the body's biochemistry, astronauts lose bone and muscle mass. He said this "has to be countered by two and a half hours of exercise each day".

"I like to say that we literally become extraterrestrials, your anatomy changes, your physiology changes, your biochemistry changes. And we still function right along, it's really quite remarkable," Dr Barratt explained.

But in the longer-term, Nasa doesn't want to just remain in low-Earth orbit. It has plans to re-visit the Moon for the first time since 1972, and then, further down the line put humans on Mars.

Dr Barratt has been helping Nasa understand the medical aspects of a round trip to Mars, which could last as much as two or three years. One of the things that will be different is the amount of radiation astronauts could potentially be exposed to.

"There's no pill you can take, there are no countermeasures, no exercise you can do like some of these other problems that we have. And so we have to very carefully monitor everybody's radiation dose," he explained.

Other medical risks involved in travelling to Mars will constitute a step change even compared with visits to the Moon.

"The difference between the Moon and Mars from a medical standpoint -from a big risk picture - is that you can actually come home from the Moon within a few days. So if you have somebody who, say, develops appendicitis, you have a fighting chance of making a diagnosis and bringing them home," Dr Barratt explained.

"If you go to Mars, there's no hope of returning in any time frame that's sensible for a medical response. So the level of autonomy that supports the same level of risk will have to be much higher. If you're going to Mars and you want the same medical risk level as in low-Earth orbit or the Moon, you will need a more highly trained medical specialist on board and much more kit so that those two are balanced."

Image source, NASA

Fellow astronauts are sending their best wishes to the crew.

There was a humorous moment when Hurley and Behnken headed down to the pad. They passed someone with a sign at the side of the road. That individual was former astronaut and SpaceX consultant Garrett Reisman who worked on the early development of Crew Dragon.

It’s a big day for SpaceX,which has ambitions of making human space travel affordable.

Today’s launch involves the Crew Dragon capsule, which will be carried into orbit on the Falcon 9 rocket, used successfully dozens of times to put satellite and cargo – but not humans – into orbit.

SpaceX is also working on a separate venture, the next generation Starship rocket, designed to one day launch humans and cargo to the moon and Mars.

The project suffered a setback on Friday, when a prototype rocket exploded during ground tests in South Texas.

With about two hours to go, the astronauts are getting settled in. There's space for four crew members, but today only two seats are taken. Doug Hurley is in seat two and Bob Behnken is in seat three. Outside, all eyes are on the weather.

Image source, NASA

The cars carrying Doug Hurley and Bob Behnken have arrived at the launch pad. The skies look blue right now, but launch remains 50-50, according to Nasa.

Image source, NASA

Image source, NASA

It's a short drive to the launch pad in the gull-wing Tesla cars.

Image source, Nasa

As Doug Hurley and Bob Behnken say farewell to their families, the microphone picks up Behnken talking to his son. He asks him: "Are you going to be on good behaviour? Are you going to listen to mummy and make her life easy?"

Then Behnken adds: "Let's light this candle." It's the phrase used by Alan Shepard, America's first astronaut, as he waited on the launch pad ahead of his historic launch on 5 May 1961.

Image source, NASA