The Polaris Dawn crew will travel 870 miles above earth, the highest altitude reached by humans since NASA’s 1972 lunar mission|@PolarisProgram|X

SpaceX has delayed its Polaris Dawn mission, which involves four private citizens—including billionaire Jared Isaacman—to Wednesday due to a helium leak in the rocket, the company said.

It is a crucial space journey as it marks the first-ever spacewalk by non-government astronauts. The mission is led by entrepreneur Isaacman, who previously flew on SpaceX’s Inspiration4 mission.

The team includes SpaceX engineers Sarah Gillis and Anna Menon, and former Air Force pilot Scott Poteet.

The crew will travel 870 miles above earth, the highest altitude reached by humans since NASA’s 1972 lunar mission, before descending to 435 miles, where Isaacman and Gillis will perform a spacewalk in SpaceX’s new spacesuits.

They will fly higher than the International Space Station, allowing research on space radiation, which is important for future missions to the Moon and Mars.