A SpaceX Falcon 9 rocket is prepared for launch of Polaris Dawn at the Kennedy Space Centre in Florida. Reuters

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 434 miles (700 km) from earth, in history's first such private spacewalk.

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

"Teams are taking a closer look at a ground-side helium leak," it added 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.

Jared Isaacman (left), Scott Poteet (right), Sarah Gillis (second left) and Anna Menon pose for a photograph.

The first US spacewalk was in 1965, aboard a Gemini capsule, and used a similar procedure to the one planned for Polaris Dawn: the capsule was depressurised, 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 US Air Force lieutenant colonel, and SpaceX employees Sarah Gillis and Anna Menon, both senior engineers at the company — will leave the spacecraft.

Riding atop a Falcon 9 rocket, the SpaceX Dragon capsule is set to reach a peak altitude of 870 miles (1,400 kilometres) – higher than any crewed mission in over half a century, since the Apollo era.

Mission commander Isaacman will guide his four-member team through the mission's centrepiece: the first-ever commercial spacewalk, equipped with sleek, newly developed SpaceX extravehicular activity (EVA) suits.

"This will be a super cool mission," SpaceX founder and CEO Elon Musk promised on X.

Joining Isaacman are mission pilot Scott Poteet, a retired US Air Force Lieutenant Colonel; mission specialist Sarah Gillis, a lead space operations engineer at SpaceX; and mission specialist and medical officer Anna Menon, also a lead space operations engineer at SpaceX.

High radiation zone

The quartet underwent more than two years of training in preparation for the landmark mission, logging hundreds of hours on simulators apart from skydiving, centrifuge training, scuba diving, and summiting an Ecuadorean volcano.

A SpaceX Falcon 9 rocket with the Crew Dragon Resilience capsule sits on Launch Complex 39A. AFP

Polaris Dawn is set to be the first of three missions under the Polaris programme, a collaboration between Isaacman, the founder of tech company Shift4 Payments, and SpaceX.

"The idea is to develop (and) test new technology and operations in furtherance of SpaceX's bold vision to enable humankind to journey among the stars," Isaacman said during a recent press conference.

Isaacman declined to reveal his total investment in the project, though reports suggest he paid around $200 million for the SpaceX Inspiration4 mission in September 2021, the first all-civilian orbital mission.

Polaris Dawn will reach its highest altitude on its first day, venturing briefly into the Van Allen radiation belt, a region teeming with high-energy charged particles that can pose health risks to humans over extended periods.

The crew will orbit nearly three times higher than the International Space Station, yet will remain far short of the record-breaking distance of over 248,000 miles set by the Apollo 13 crew in 1970.

Apollo 13's astronauts journeyed that far to slingshot around the far side of the Moon after an explosion crippled their spacecraft, aborting their planned lunar landing and necessitating a return to Earth without major propulsive manoeuvres.

New spacesuits

On day three, the crew will don their state-of-the-art EVA spacesuits – outfitted with heads-up displays, helmet cameras, and advanced joint mobility systems – and take turns to venture outside their spacecraft in twos. Each will spend 15 to 20 minutes in space, 435 miles above Earth's surface.

Notably, however, even the pair strapped into their seats will be exposed to the vacuum of space as the Dragon capsule doesn't have an airlock.

The following day will be dedicated to testing laser-based satellite communication between the spacecraft and Starlink, SpaceX's more than 6,000-strong constellation of internet satellites, in a bid to boost space communication speeds.

The crew is also set to conduct nearly 40 experiments aimed at advancing our understanding of human health during long-duration spaceflights. Among these are tests with contact lenses embedded with microelectronics to continuously monitor changes in eye pressure and shape.

After six days in space, the mission will conclude with a splashdown off the coast of Florida, where a SpaceX recovery ship will await.

The second Polaris mission will also utilise a Dragon capsule, while the third and final mission is slated to be the maiden crewed flight of Starship, SpaceX's prototype next-generation rocket that is key to Musk's vision of one day colonising Mars.

Agence France-Presse