The launch of SpaceX's four-person Polaris Dawn mission will be delayed by at least a day because of a helium leak in ground equipment at Kennedy Space Center, the company said on Tuesday, hours before the scheduled liftoff of its Crew Dragon capsule.

The highlight of the five-day mission is expected to come two days after launch when the crew embarks on a 20-minute spacewalk 700 kilometers from Earth in history's first such private spacewalk.

The company now aims to launch the spacecraft, carried by a Falcon 9 booster, at 3:38 a.m. local time (0738 GMT) on Wednesday, it said in a post on X.

"Teams are taking a closer look at a ground-side helium leak," it said in Tuesday's post. "Falcon and Dragon remain healthy, and the crew continues to be ready for their multi-day mission to low-Earth orbit."

Only government astronauts have performed spacewalks to date, most recently by occupants of the International Space Station, who regularly don spacesuits to perform maintenance and other checks of their orbital home.

The first U.S. spacewalk was in 1965 aboard a Gemini capsule and used a similar procedure to the one planned for Polaris Dawn: the capsule was depressurized, the hatch opened, and a spacesuited astronaut ventured outside on a tether.

Polaris Dawn's crew will be testing SpaceX's new slimline spacesuits during the spacewalk.

Only two of the four – billionaire Jared Isaacman, mission pilot Scott Poteet, a retired U.S. Air Force lieutenant colonel, and SpaceX employees Sarah Gillis and Anna Menon, both senior engineers at the company – will leave the spacecraft.

Isaacman, the founder of electronic payment company Shift4, bankrolled the mission; he has declined to say how much he has spent, but it is estimated to be more than $100 million.