SpaceX is preparing to send the first all-civilian crew into Earth's orbit Wednesday evening, capping a summer of private spaceflight with one of the most ambitious tourism missions to date.
A five-hour launch window for "Inspiration4" opens from 8:02 pm (0002 GMT Thursday), and weather conditions remain good with an 80 percent chance of launch, according to official forecasters.
After a visit from SpaceX boss Elon Musk, the crew climbed into white Teslas to applause from a small crowd under sunny skies, and headed to the facility where they will don their spacesuits.
A Falcon 9 rocket, with a Dragon capsule at its top, will blast off from the legendary launch complex 39A at NASA's Kennedy Space Center in Florida, where the Apollo 11 mission once took off for the Moon.
The spaceship's trajectory will take it to an altitude of 357 miles (575 kilometers), which is deeper into space than the International Space Station (ISS).
After spending three days orbiting the globe, the four-person crew, all Americans, will splash down off the Florida coast.
"The #Inspiration4 launch reminds us of what can be accomplished when we partner with private industry!" tweeted NASA administrator Bill Nelson ahead of the launch.
Building up commercial capability has been the vision of NASA's commercial crew program since it was founded in 2011.
- Tough training -
The trip was paid for by billionaire Jared Isaacman, a 38-year-old high school dropout, founder of Shift4 Payments, and aviator.
SpaceX hasn't disclosed what it cost him, but the price tag runs into tens of millions of dollars. Isaacman's three crewmates were selected through a competition, and their stories have been followed in a Netflix documentary.
Hayley Arceneaux, a pediatric cancer survivor, is a 29-year-old physician assistant. She will be the youngest American to go into orbit and the first person with a prosthesis, on a part of her femur.
Chris Sembroski, 42, is a US Air Force veteran who now works as an aerospace data engineer.
Sian Proctor, a 51-year-old geoscientist and educator, was almost selected to become an astronaut for NASA in 2009.
She will be only the fourth African-American woman to go to space.
They bonded over the course of six months' training that included climbing Mount Rainier, high G-force conditioning, and experiencing a taste of weightlessness on a parabolic flight.
The mission is hoping to raise $200 million for St Jude's Children's Research Hospital, a leading facility in Tennessee. Arceneaux received treatment there as a child, and now works there.
The crew will take with them various objects -- a ukulele, 66 pounds (30 kilograms) of hops intended to brew space beer and several digital assets known as non-fungible tokens -- that will be auctioned off for the cause.
Throughout the flight, biological data including heart rate and sleep, as well as their cognitive capacities, will be analyzed to study the health impacts of space.
The Dragon will be equipped, for the first time, with a cupola observation dome -- the largest ever space window -- to take in the view. The dome replaces the usual mechanism used on Dragons to dock with the ISS.
- Privatization of space -
Beyond the charitable and scientific aspects, the mission's stated goal is to represent a turning point in the democratization of space, by proving that the cosmos is accessible to people who have not been handpicked and trained for many years as astronauts.
For SpaceX, this is nothing less than a first step towards a multi-planetary humanity -- founder and CEO Musk's ultimate vision.
The flight should remain fully automated, but the crew has been trained by SpaceX to be able to take control in the event of an emergency.
The space adventure bookends a summer marked by the battle of the billionaires Richard Branson and Jeff Bezos to reach the final frontier.
Branson, the Virgin Galactic founder, achieved the feat first, on July 11, and was followed by the Blue Origin boss nine days later.
But these flights only offered a few minutes of weightlessness. SpaceX's mission is far more ambitious -- though flights organized by a private company that contracted Russian Soyuz rockets in the 2000s took tourists to the ISS.
This will be the fourth crewed mission for SpaceX, which has now sent 10 astronauts to the ISS for US space agency NASA.