CAPE CANAVERAL, Fla.—After beating bone cancer, Hayley Arceneaux figures rocketing into orbit on SpaceX’s first private flight should be a piece of cosmic cake. St. Jude Children’s Research Hospital announced Monday that the 29-year-old physician assistant—a former patient hired last spring—will launch later this year alongside a billionaire who’s using his purchased spaceflight as a charitable fundraiser. Arceneaux will become the youngest American in space—beating NASA record-holder Sally Ride by over two years—when she blasts off this fall with entrepreneur Jared Isaacman and two yet-to-be-chosen contest winners. She’ll also be the first to launch with a prosthesis. When she was 10, she had surgery at St. Jude to replace her knee and get a titanium rod in her left thigh bone. She still limps and suffers occasional leg pain, but has been cleared for flight by SpaceX. She’ll serve as the crew’s medical officer. “My battle with cancer really prepared …