The weather seems cooperative and the passengers are raring to go: SpaceX's first space tourists were a picture of calm Tuesday as they prepared to blast off on a three-day mission to orbit the Earth.

"No jitters, excited to get going!" American billionaire Jared Isaacman told reporters when asked his state of mind at a press conference.

Behind him towered the giant Falcon 9 rocket, which will carry the four-member, all civilian crew for the mission, dubbed Inspiration4.

"Any jitters are the good kind," added 29-year-old physician assistant Hayley Arceneaux. "I'm just so excited for tomorrow to get here." 

Chris Sembroski, a 42-year-old Air Force veteran, even compared the extraordinary voyage to a "camper van" ride.

He's not far off: the interior of the Dragon Capsule offers its occupants a cramped eight by four meters (27 by 13 feet) of space.

"You just wrap yourself in (sleeping bags) so you don't float into each other during the middle of the night!" he joked.

They all said they got along well, even though they didn't know each other before they started training only about six months ago. 

Isaacman, who chartered the mission at his expense, ran a competition to choose the three joining him.

"Since the announcement... every day has been the best day of my life, and it's only getting better," declared Sian Proctor, a 51-year-old geology professor who will become the first African American woman to assume the role of "pilot" on a spaceflight.

Clearly moved by the occasion, she shared that she and the crew had had a phone chat with former first lady Michelle Obama.

"We had a very nice conversation that will stay with you for the rest of my life." 

A take-off window begins at 8:02pm Wednesday (0002 GMT Thursday).

"Right now, the weather is trending well," said Benji Reed, chief of crewed missions for SpaceX.