image copyrightMars Scientificimage captionSunday will be the first time the ship has carried four passengers in the back
UK businessman Sir Richard Branson is about to realise a lifetime's ambition by flying to the edge of space.
He'll ride his Virgin Galactic rocket plane on Sunday to an altitude where the sky turns black and the Earth's horizon curves away into the distance.
The entrepreneur says he wants to evaluate the experience before allowing paying customers aboard next year.
The vehicle will set off for the 1.5-hour mission above New Mexico at about 07:00 local US time (14:00 BST).
Virgin Galactic will be providing an online stream of the event.
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media captionWatch: The carrier plane releases Unity to begin a test ascent
It's been a long road for Sir Richard to get to this point. He first announced his intention to make a space plane in 2004, with the expectation he'd have a commercial service available by 2007.
But technical difficulties, including a fatal crash during a development flight in 2014, have made the space project one of the most challenging ventures of his career.
image copyrightVirgin Galacticimage captionBranson will be accompanied by two pilots and three crewmates from the company
"I've wanted to go to space since I was a kid, and I want to enable hopefully hundreds of thousands of other people over the next 100 years to be able to go to space," Sir Richard told the BBC.
"And why shouldn't they go to space? Space is extraordinary; the Universe is magnificent. I want people to be able to look back at our beautiful Earth and come home and work very hard to try to do magic to it to look after it."
How does his rocket plane work?
The vehicle, known as Unity, will be carried by a much bigger aeroplane to an altitude of about 15km (50,000ft), where it will be released.
A rocket motor in the back of Unity will then ignite and blast the ship skyward. The motor will burn for 60 seconds, by which time Sir Richard, his three crewmates and the two pilots up front, will have an extraordinary view of the planet below.
The maximum height achievable by Unity is roughly 90km (50 miles, or 295,000ft), but towards the top of the climb Sir Richard will start to enjoy a few minutes of weightlessness and he'll be able to float around the cabin and to look out of the window.
Eventually, though, he'll have to strap back into his seat for the glide return back to the spaceport in New Mexico.
What will he see from the window?media captionBeth Moses: "It stuck in my soul; it's totally beautiful"
Sir Richard will be taking instruction throughout the flight from Beth Moses. She's the chief astronaut instructor at the businessman's Virgin Galactic company. Apart from the firm's cadre of test pilots, Moses is the only person who's so far experienced the exhilaration of an ascent. The view out of the window, she says, is "just phenomenal".
image copyrightPA Mediaimage captionThe long road: Sir Richard initiated his Virgin Galactic project in 2004
"Pictures don't do it justice. It's just so bright and beautiful. I saw the ocean, and halfway up the US and halfway down into Mexico. I saw the green of the land and the white snow-capped mountains," she told BBC News.
"Because you are weightless and still, and the ship has come to a stop, you can just soak it in, in a really timeless way. It stuck in my soul."
Who is Sir Richard's competition?image captionUnity folds its tailbooms on descent to stabilise its fall before then gliding home
Unity is a sub-orbital vehicle. This means it can't achieve the velocity and altitude necessary to keep it up in space to circle the globe.
The only other near-market sub-orbital system belongs to Amazon.com founder, Jeff Bezos. He has a rocket and capsule he calls New Shepard, and he will fly on its inaugural crewed flight on 20 July.
The retail billionaire is going to ride to just over 100km above Texas, alongside his brother, Mark; the famed female aviator Wally Funk; and a mystery individual who bid $28m (£20m) in a ticket auction.
But while Sir Richard has a line of some 600 individuals who've already paid deposits for tickets priced at up to $250,000 (£180,000), Mr Bezos has said little yet about how he intends to commercialise New Shepard.
Is the competition friendly?image copyrightBlue Originimage captionNew Shepard has no pilots. The flight is fully computer-controlled
Sir Richard says he has spoken to Jeff Bezos on the phone and they have wished each other well in their space endeavours. But there is no doubting there is some edge in the relationship.
Mr Bezos was the first to announce his mission, only to see Sir Richard then move up his own published schedule so he could win first-flight bragging rights.
On Friday, Mr Bezos's Blue Origin space company issued a tweet that took a pop at Virgin Galactic's Unity vehicle. The posting repeated a claim that anyone who flew on the rocket plane would forever have an asterisk by their name because they wouldn't reach the "internationally recognised" altitude for where space begins – the so-called Kármán line of 100km.
The tweet also said Unity's impacts on the environment were far greater than New Shepard's. Virgin Galactic told the BBC that the carbon footprint of flying in Unity is equivalent to a business flight from London to New York, but that all the company's activities are offset.
From the beginning, New Shepard was designed to fly above the Kármán line so none of our astronauts have an asterisk next to their name. For 96% of the world’s population, space begins 100 km up at the internationally recognized Kármán line. pic.twitter.com/QRoufBIrUJ
— Blue Origin (@blueorigin) July 9, 2021
The BBC is not responsible for the content of external sites.View original tweet on Twitter
The US government recognises the boundary of space to be at about 80km (50 miles) and awards astronaut wings to anyone who exceeds this altitude.
media captionFive things to know about Amazon boss Jeff Bezos