Led by the fintech billionaire Jared Isaacman, the SpaceX Polaris Dawn mission was launched early on Tuesday from the Kennedy Space Center in Florida, reaching a maximum altitude of 870 miles (1,400 kilometers).
That's more than three times higher than the International Space Station, in a region of space known as the inner Van Allen radiation belt-a zone teeming with dangerous, high-energy particles.
With the Dragon spaceship's elliptical orbit now ranging from an estimated 120-mile low to a high of around 430 miles, the crew of four is preparing for the centerpiece of the mission: an audacious EVA, scheduled for 0958 GMT on Thursday with a backup window on Friday.
SpaceX had pushed the time back by a few hours early Thursday, without explaining why. It planned a webcast of the event starting about an hour beforehand, on its website.
'Bit of a dance'
First, before the hatch opens, the crew does a "prebreathe" to remove the nitrogen from their blood to avoid decompression sickness from nitrogen bubbles. Then, cabin pressure is slowly released to space-level pressure.
Once opened, Isaacman and his fellow crew member Sarah Gillis, a SpaceX engineer, will be allowed to pop their heads out one at a time from a structure attached to the hatch called "Skywalker" - fitted with hand and footholds.
"It's going to look like we're doing, a little bit of a dance," Isaacman joked during a recent press conference.
In fact, the crew was stress-testing SpaceX's next-generation suits kitted out with heads-up displays, helmet cameras, and better joint mobility systems.
But they won't be floating off on a tether like the early spacefarers did, such as Soviet cosmonaut Alexei Leonov or NASA's Ed White in 1965. Instead, they will be hanging on for dear life as the spacecraft zooms around Earth at about 17,500 mph.
Because the Crew Dragon doesn't have an airlock, the entire crew will stay in the vacuum of space during the spacewalk, which is expected to take roughly two hours.
After the hatch is closed, the cabin will re-pressurize, and oxygen and nitrogen levels will return to normal.
Mission pilot Scott Poteet and SpaceX engineer Anna Menon will monitor vital support systems during the activity, while Isaacman and Gillis are expected to each spend about 15 to 20 minutes partially outside the craft.
"The risk is greater than zero, that's for sure, and it's certainly higher than anything that has been accomplished on a commercial basis," Sean O'Keefe, who served as NASA administrator from 2001 to 2005, told AFP.
"This is another watershed event in the march toward commercialization of space for transportation," he said, likening the crewmates to the pioneering aviators who made air travel commonplace.
First of three Polaris missions
All four had been training for more than two years for this momentous voyage, putting in hundreds of hours on simulators plus skydiving, centrifuge training, scuba diving, and climbing an Ecuadoran volcano.
Besides their spacewalk, the crew will be testing satellite communications via laser beams between the spacecraft and the huge Starlink satellite constellation.
They also will conduct 36 science experiments, among them tests on contact lenses with embedded microelectronics to monitor changes in eye pressure and shape in space.
Polaris Dawn is the first of three missions under the Polaris program, a partnership between Isaacman and SpaceX.
Financial terms of the partnership remain undisclosed, but Isaacman, a 41-year-old founder and chief executive of Shift4Payments, had reportedly invested $200 million of his fortune to lead the first all-civilian SpaceX Inspiration4 orbital mission back in 2021.
This summer's Polaris Mission had been billed as the maiden crewed flight of SpaceX's Starship-a prototype next-generation rocket that is central to founder Elon Musk's aims of colonizing Mars.
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