Four civilians are due to blast off into space on Wednesday, weather permitting, in the world's first orbital excursion manned only by amateur astronauts.
The flight is called Inspiration4 and marks a giant step in space tourism. It is being run by Elon Musk's SpaceX and is funded by Jared Isaacman, the billionaire founder of Shift4 Payments.
The Falcon 9 rocket carrying the SpaceX Dragon capsule has a five-hour window for launch starting at 8:02 pm on Wednesday (0102 GMT on Thursday) from Florida's Kennedy Space Center.
It will then orbit Earth for three days before splashing down off the coast of the southern US state.
Three passengers are to join Isaacman on the first all-civilian human spaceflight: Hayley Arceneaux, doctor's assistant who beat bone cancer as a child; Chris Sembroski, an Air Force veteran and aerospace data engineer; and geoscientist Sian Proctor.
Amateurs have gone into space before, but never without the presence of trained astronauts or cosmonauts.
Isaacman, an experienced jet pilot, announced his plans in February, saying he had loved space since he was a child.
During the trip, the Inspiration4 crew plans to carry out health research they hope will be useful to future long-duration spaceflights, according to a SpaceX statement.
It is unclear how much Isaacman paid to charter the four-seat flight.
Isaacman and SpaceX describe it as a charitable mission that seeks to raise 200 million dollars for St Jude Children's Hospital and Research Center in Memphis, Tennessee. Arceneaux was treated there for bone cancer as child - and it is where she now works.
Isaacman committed 100 million dollars to St Jude and picked Arceneaux as his guest. The two other seats were sold in a fundraising raffle for St Jude.
Inspiration4 will fly higher and farther than space flights recently funded by entrepreneurs Richard Branson and Jeff Bezos, whose trips were relatively brief, and below the 100 kilometre altitude level.
The Inspiration4 mission is due to orbit at some 580 kilometres above Earth, higher than the International Space Station. It is to orbit the planet at 30,000 kilometres per hour every 90 minutes.
Even if the Dragon spacecraft is virtually self-steering, the trip is not entirely without risk. However, the Inspiration4 mission is seen as key stage in the transition towards private space flight.