A four-person crew for SpaceX's Polaris Dawn mission arrived in Florida on Monday ahead of their August 26 launch to space for a mission that includes the first privately managed spacewalk, a risky endeavor only government astronauts have done in the past.

The crew -- a billionaire entrepreneur, a retired military fighter pilot and two SpaceX employees -- neared the end of more than two years of training for the mission, in which they will venture out of their Crew Dragon capsule in Earth's orbit for a tethered spacewalk.

The mission will be a major first test of SpaceX's new astronaut spacesuits and marks the latest risky, high-stakes commercial milestone that Elon Musk's space company is looking to clinch on the billionaire's path to eventually building colonies on Mars.

"Whatever risk associated with it, it is worth it," said mission commander Jared Isaacman, the CEO of electronic payment company Shift4 who is also the head of the SpaceX-affiliated Polaris program.

"We have no idea what it could do to really change the trajectory of humankind ... there has to be some first steps in this direction," Isaacman told reporters on Monday during a news conference.

Isaacman is bankrolling the mission and others under his Polaris program. He declined to say how much he has spent on the missions so far, but it's a total that would be hundreds of millions of dollars.

The launch is scheduled for 3:38 a.m. ET (0738 GMT) on August 26 from SpaceX's launchpad at NASA's Kennedy Space Center in Florida. The mission is expected to last six days, with the spacewalk -- formally called Extravehicular Activity (EVA) -- planned for the third day.

The rest of the Polaris Dawn crew includes mission pilot Scott Poteet, a retired US Air Force Lieutenant Colonel who was also on the Inspiration4 mission.

SpaceX employees Sarah Gillis and Anna Menon, both Lead Space Operations Engineers at the company, will be mission specialists.

Crew Dragon has no airlock, so its entire cabin will be slowly depressurised ahead of the spacewalk, meaning all four astronauts will be testing out the new spacesuits. But only two, Isaacman and Gillis, will float outside the spacecraft.

Only government astronauts from the US, former Soviet Union and Russia, the European Space Agency, Canada and China have conducted spacewalks. Using American and Russian spacesuits, over 270 spacewalks have been conducted outside the International Space Station since its inception in 2000.

"EVA is a risky adventure. But again, we did all the work to really get ready for this," said Gerstenmaier, who was NASA's human spaceflight chief until 2020.

"We kind of built off of what NASA's heritage was, but I think we've also extended NASA's heritage a little bit further," Gerstenmaier said.

While SpaceX uses its Crew Dragon capsule to send astronauts to and from the ISS for NASA, the company has sought to arrange privately funded spaceflights that feature new milestones with each mission.

The first mission led by Isaacman, Inspriration4 in 2021, was the first all-civilian, privately funded flight into Earth's orbit. SpaceX this month said it plans to launch next year the first crew to ever orbit the Earth pole-to-pole, featuring a multinational crew.