Jared Isaacman, the billionaire founder of payments company Shift4 who flew on the first private SpaceX flight to orbit last year, has purchased as many as three more flights from Elon Musk’s company.

The first mission in the so-called Polaris Program is set to launch a four-person crew led by Isaacman in the fourth quarter with the company’s Falcon 9 rocket and Crew Dragon spacecraft. According to the program’s website, the augural flight, Polaris Dawn, will be the first of up to three missions, with the final one to be the first crewed spaceflight of SpaceX’s Starship rocket.

“The Polaris Program is an important step in advancing human space exploration while helping to solve problems through the use of innovative technology here on Earth,” Isaacman said in a statement.

Financial terms of the billionaire’s purchase from SpaceX were not disclosed.

Isaacman was among those who were at Musk’s Starship presentation last week at the company’s facility in Texas. SpaceX has booked a private Starship flight to the moon for Japanese billionaire Yusaku Maezawa, and Musk on Thursday alluded that more such spaceflights were in the works but didn’t offer any specific details.

The Polaris Dawn mission will spend up to five days in orbit and is looking to achieve such goals as the first private-company spacewalk with SpaceX spacesuits, testing of Starlink satellite communications in space and conducting scientific research on human health.

Notably, Polaris Dawn marks the start of SpaceX’s own astronaut corps. Isaacman, the mission’s commander, will be joined by longtime colleague Scott Poteet as the pilot. Two SpaceX employees, Sarah Gillis and Anna Menon, the company’s lead space operations engineers, round out the crew. Gillis oversees the astronaut-training program and Menon manages the development of its crew operations.

Isaacman led the historic Inspiration4 mission in September, which spent three days in orbit in SpaceX’s Crew Dragon capsule with a crew of four. The primary goal of Inspiration4 was to raise $200 million for St. Jude Children’s Research Hospital; it surpassed that goal and donated over $240 million to the charity. Like Inspiration4, the Polaris missions will “raise funds and awareness” for St. Jude, according to the program’s website.

The program is also collaborating with multiple organizations, including the Translational Research Institute for Space Health (TRISH), BioServe Space Technologies at the University of Colorado Boulder, Space Technologies Lab at Embry-Riddle Aeronautical University, Weill Cornell Medicine, Johns Hopkins University Applied Physics Laboratory, the Pacific Northwest National Laboratory and the U.S. Air Force Academy.