Tonight’s historic SpaceX launch is set to go ahead despite more thunderstorms on the forecast.
Elon Musk’s company came within 17 minutes of launching on Wednesday, before the threat of lightning forced a delay.
Weather forecasters have set the odds of acceptable conditions at 50-50 for this evening’s feat, but NASA says it’s pressing ahead anyway.
NASA administrator Jim Bridenstine tweeted to say Saturday’s launch preparations were moving forward, but added: “Weather challenges remain with a 50 per cent chance of cancellation.”
He later warned thunderstorms are expected later and there is a 50-50 chance of lift-off.
He said: “What got us last time (Wednesday’s launch) was the electricity in the atmosphere. And of course, today there are, in fact, build-ups.
“It doesn’t look like there are thunderstorms at this time but they are expected. The question is when do those thunderstorms go away and when those thunderstorms materialise where are they located.”
He added: “We are predicting about a 50-50 shot of going this time but given the fact that we are in late May in Florida we have to take every shot we can get.
Mr Bridenstine said he was hoping that the weather will hold up, adding: “We are go for launch right now.”
He said: “We almost made it on Wednesday and the trend is better today than it was on Wednesday.”
Elon Musk, who is the chief executive of SpaceX, said the teams were proceeding with countdown, despite the weather risk.
If the launch is delayed, the next available window will be on May 31 at 8pm UK time.
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The two astronauts undertaking the mission, Doug Hurley and Bob Behnken, were pictured beaming as they set out for the launch pad.
Mr Behnken pantomimed a hug of his 6-year-old son, Theo, and said: "Are you going to listen to Mommy and make her life easy?"
Meanwhile, Mr Hurley blew kisses to his 10-year-old son and wife.
Because of the coronavirus outbreak, NASA has severely limited the number of employees, visitors and journalists allowed deep inside Kennedy Space Centre.
The space agency urged people to stay safe and watch from home, and by NASA's count, at least 1.14 million viewers followed the launch preparation online.
But spectators also began lining the Cape Canaveral area's beaches and roads. Signs along the main beach drag read, "Godspeed."
The International Space Station - In pictures
Since ending its Space Shuttle programme in 2011, NASA has depended on Russia’s space agency Roscosmos to transport its astronauts to the space station.
In 2014, NASA awarded SpaceX and Boeing contracts to provide crewed launch services to the space station as part of its Commercial Crew Programme.
According to NASA, the aim of the latest mission, named Demo-2, is to show SpaceX’s ability to ferry astronauts to the space station and back safely.
It is the final major step required by SpaceX’s astronaut carrier, the Crew Dragon, to get certified by NASA's Commercial Crew Programme for more long-term manned missions to space.
SpaceX’s Falcon 9 rocket will take off from the Launch Complex 39A at the Kennedy Space Centre in Florida, carrying the Crew Dragon spacecraft where Mr Behnken, 49, and Mr Hurley, 53, will be strapped in.
Mr Behnken will serve as the mission’s joint operations commander and take responsibility for the rendezvous, docking and undocking of the Dragon capsule, while Mr Hurley will be in charge of the launch, landing and recovery of the vehicle in his role as the Crew Dragon spacecraft commander.
Around 10 minutes after lift-off, the rocket will separate into a first stage and a second stage.
The first stage will return to a SpaceX landing ship which will be stationed in the Atlantic Ocean off the coast of Florida, while the second part of the rocket continues the journey with the Crew Dragon.
Once in orbit, the Crew Dragon will separate from the second stage and travel at around 17,000mph.
The craft is expected to rendezvous and dock, with the space station on Sunday at 3.30pm UK time.
Mr Behnken and Mr Hurley will join the three other space station residents – Nasa’s Chris Cassidy and Russia’s Anatoli Ivanishin and Ivan Vagner – to become members of the Expedition 63 crew.
Mr Vagner tweeted earlier to say he was waiting for the duo at the space station, before sharing an update that the pair were given the to wear their spacesuits.
“We are GO for suit up! #LaunchAmerica Liftoff is slated for today at 3:22 pm (20:22BST),” he wrote.
The mission is expected to last anything between one and four months, with a number of tests being performed on the Crew Dragon.
All being well, the launch will take place at 8.22pm UK time (3:22pm local time).
If successful, it will be the first launch from the US with NASA astronauts for nearly a decade.