A mystery bidder paid $28 million at auction on Saturday for a seat alongside Jeff Bezos on board the first crewed space flight by the billionaire's Blue Origin company next month.
The Amazon founder revealed last week that he and his brother Mark would take seats on board the company's New Shepard launch vehicle on July 20, to fly to the edge of space and back.
The Bezos brothers will be joined by the winner of Saturday's charity auction, whose identity remains unknown, and by a fourth, as yet unidentified space tourist.
"The name of the auction winner will be released in the weeks following the auction's conclusion," Blue Origin tweeted after the sale.
"Then, the fourth and final crew member will be announced – stay tuned."
Saturday's successful bidder beat 20 rivals in a May 19 auction that wrapped up with a 10-minute bidding frenzy.
Bidding reached $4.8m by Thursday, but shot up spectacularly in the final live auction, rising by million-dollar increments.
The proceeds, aside from a 6 per cent auctioneer's commission, will go to Blue Origin's foundation, Club for the Future, which aims to inspire future generations to pursue careers in Stem subjects – science, technology, engineering and mathematics.
Taking off from a desert in western Texas, the New Shepard trip will last 10 minutes, four of which passengers will spend above the Karman line that marks the recognised boundary between Earth's atmosphere and space.
After lift-off, the capsule separates from its booster, then spends four minutes at an altitude of more than 100 kilometres, during which time those on board experience weightlessness and can observe the curvature of Earth.
The booster lands autonomously on a pad 3km from the launch site, and the capsule floats down to the surface with three large parachutes that slow it down to a speed of about 1.5kph when it lands.
Mr Bezos, who announced this year that he is stepping down as Amazon's chief executive to spend more time on other projects, including Blue Origin, said it was a lifelong dream to fly into space.
The New Shepard has carried out more than a dozen successful uncrewed test runs from its facility in the Guadalupe Mountains of Texas.
"We're ready to fly some astronauts," Blue Origin's director of astronaut and orbital sales, Ariane Cornell, said on Saturday.
The reusable suborbital rocket system was named after Alan Shepard, the first American in space 60 years ago.
The automated capsules have six seats with horizontal backrests placed next to large portholes, in a futuristic cabin with fancy lighting. Cameras record the few minutes the space tourists experience weightlessness.
Blue Origin's maiden crewed flight comes amid fierce competition in the field of private space exploration, with Elon Musk's SpaceX, and Virgin Galactic, founded by British billionaire Richard Branson, all jostling for pole position.
Mr Bezos has a very public rivalry with Mr Musk, whose SpaceX is planning orbital flights that would cost millions of dollars and send people much further into space.
SpaceX has already begun to carry astronauts to the International Space Station and is a competitor for government space contracts.
Virgin Galactic, meanwhile, hopes to begin regular commercial suborbital flights early next year, with eventual plans for 400 trips a year.
About 600 people have booked seats, costing from $200,000 to $250,000, and there has been talk of Mr Branson himself taking part in a test flight this summer, although no date has been set.