Rocket company SpaceX will make a second attempt in the coming hours to get Nasa astronauts Doug Hurley and Bob Behnken into orbit.
Their flight to the International Space Station (ISS) on Wednesday was postponed because of poor weather at Florida's Kennedy Space Center.
Saturday's forecast is not brilliant. The meteorologists say conditions will probably be touch and go again.
The lift-off is scheduled for 15:22 EDT (19:22 GMT; 20:22 BST).
There's huge focus on Hurley's and Behnken's mission because it will mark the first time that the United States has been able to launch its astronauts to the ISS since the retirement of Nasa's space shuttles in 2011.
It will also be the first occasion that the US space agency has used a private company to transport one of its crews to orbit.
But people were wrong if they thought this attention added to the pressure to get the astronauts off the ground, said Nasa Administrator Jim Bridenstine.
"We will launch when we are ready," he told reporters. "I'll tell you, the President (Donald Trump) and the Vice President (Mike Pence) were proud of the Nasa team and the SpaceX team for making the right call for the right reasons (on Wednesday)."
For skywatchers in the UK, the Saturday flight is less favourable for catching sight of the capsule coming overhead. The first pass following lift-off comes when conditions are too light (just after 20:40 BST). The second pass, at around 22:15 BST, is better, but the vehicle will be very low on the horizon in the southwest.
The first launch attempt in the week was scrubbed just 16 minutes before the designated launch time. There had been much electrical activity in the air throughout the day, and controllers concluded it wouldn't be prudent to proceed with the flight.
At the moment of postponement, Hurley and Behnken were sitting in their Dragon capsule atop its Falcon rocket with the booster fuels being loaded below them.
The frustration was that the countdown was going so smoothly; engineers had seen no technical issues of concern. The vehicles were in perfect shape to begin their ascent.
Saturday will follow exactly the same routines. The astronauts will head out to the pad about three hours prior to 15:22 EDT. A SpaceX "close out" team will help them strap into their capsule seats, and then it will be a case of running through the pre-flight checks with controllers.
Nasa astronaut Nicole Mann said her colleagues were unflustered by the potential for further delay.
"There are plenty of things in life you can't control, the weather being one of them," she said. "You need just to remain flexible, not to waste any energy on those things you can't control. And then do what you need to do: prepare, and then when it's time for the next launch opportunity, you know you're ready to go."
If the launch goes ahead, Hurley and Behnken have about a 19-hour flight to the ISS. They'll use that time to try out systems onboard the Dragon capsule, including having a go at manual flying. They'll also need to get some sleep after what will have been a long day.
The duo are expected to stay at the ISS for between one and four months before returning to Earth.
SpaceX, which is run by the tech billionaire Elon Musk, has a $2.6bn contract with Nasa to provide six crew flights to the space station. The first of these is scheduled for the end of August, assuming nothing untoward happens on Hurley's and Behnken's demonstration.
The Boeing company has a similar contract, but it is a year at least behind SpaceX in its development timeline.