Sir Richard Branson plans to make astronautical history this weekend by becoming the first billionaire in space
If his schedule holds, the 70-year-old Virgin Galactic founder will lift off on Sunday afternoon (around 1am Monday NZT - Virgin Galactic will be livestreaming the event) from a runway near the small town of Truth and Consequences in New Mexico on the maiden passenger flight of his company's SpaceShipTwo spaceplane.
Dubbed the VSS Unity, it will be lifted to about 50,000 feet (15,000m) by a specially made aeroplane, WhiteKnightTwo, before detaching from the mothership, firing its hybrid rocket engine and climbing beyond Earth's atmosphere into space.
Branson will then enjoy a few minutes of weightlessness and unbeatable views through Unity's 12 windows, before re-entering the atmosphere and gliding back down to a conventional landing between one and two hours after launch.
"At that moment, we will have become astronauts," Branson told a US TV crew last week.
"I will pinch myself. And pinch myself again and again. I can't wait."
This is the experience that Virgin hopes to begin selling to space tourists as early as next year, with 700 customers already having bought tickets for between $US200,000 and $250,000 ($NZ285,000-$360,000).
If the mission is successful, Branson will narrowly beat his rival Jeff Bezos, the richest person in the world, who recently stepped down as Amazon chief executive to focus on other projects such as his space exploration company Blue Origin.
Bezos and his brother Mark will blast off in a more conventional two-stage rocket called New Shepard on July 20.
Branson and Bezos are vying to capture the nascent space tourism industry, which will be worth around $3.4 billion by 2028, according to analysts.
On Friday, Blue Origin hit back at its competitor by claiming that Branson will not be a real astronaut because he will not be going above the Karman line, which some scientists use to define the boundary of outer space.
The company said: "From the beginning, New Shepard was designed to fly above the Karman line so none of our astronauts have an asterisk next to their name. For 96 per cent of the world's population, space begins 100km up at the internationally recognised Karman line."
The US Air Force and Federal Aviation Administration (FAA) define space as beginning lower, at 80km. Branson claims he does not see himself as a rival to Bezos and that Blue Origin's flight played no role in Virgin's schedule.
Another competitor is SpaceX, whose founder Elon Musk, another billionaire, has said there is a 70 per cent chance in the future he will personally live on Mars. The company has already become an indispensable workhorse for Nasa, the cash-strapped US space agency, in resupplying the International Space Station (ISS).
Sunday's launch will be a moment of truth for Branson's 17-year vision, which has eaten up more than $1b of his money, lagged well behind schedule and been dogged by delays, technical errors and sometimes fatal accidents.
In 2014, a test flight operated by a contractor ended in tragedy when Unity's sister craft, Enterprise, disintegrated in mid-flight, killing its co-pilot. In 2007, the same contractor suffered an accidental explosion during engine tests that killed three people.
"Everybody tests, and the question is always did you test enough? Did you test correctly? Did you miss something in your test?" said Clayton Anderson, a former Nasa astronaut. "Statistically, something bad is going to happen over time, and when that time comes what will they do next?
He added: "There are three Ds of space flight: danger, difficulty and dollars. Apparently they have the dollars. I think they understand the danger. The question is how they handle the difficulty...
"They'll get to see the Earth from above; they'll ooh and ahh and it will probably change their perspective on life. But I challenge them to spend five, six, seven, eight, nine, ten months in space. And then I challenge them to spend it on a trip to Mars, which is something I don't really want to do."
Branson and Bezos's flights will be merely "sub-orbital", meaning they will not finish a full circuit off the Earth. The Unity is designed as a luxury product, with reclining seats, automated mood lighting, a mirror to watch yourself float, snazzy blue flight suits by fitness brand Under Armour, and 16 cameras recording high-definition souvenir footage.
Andrew Chanin, head of the space-focused index fund Procure Space, said investors were encouraged by Branson taking such an unusual risk. He said: "I'd call it more than a publicity stunt, there's no trickery going on ... this goes a long way in providing comfort to would-be space tourists."