Jared Isaacman is leading a series of SpaceX missions called the Polaris Program.Patrick T. Fallon/AFP/Getty ImagesThe billionaire Jared Isaacman said his Polaris missions with SpaceX are "a question mark" now.Donald Trump nominated Isaacman for NASA Administrator months after he did SpaceX's first spacewalk.Space experts doubt Isaacman will fly during his NASA term, due to job demands and safety risks.SpaceX and its go-to billionaire-turned-private-astronaut seem to be going their separate ways, at least for the next four years.Jared Isaacman has flown two SpaceX missions to space and is slated to fly two more.However, Isaacman may no longer fly those missions now that President-elect Donald Trump has tapped him to lead NASA.Isaacman is the founder and CEO of a payments-processing company called Shift4, but he's more famous for conducting the world's first commercial spacewalk in September.