Mr. Musk’s time in Washington has brought significant benefits to his fastest-growing company, SpaceX, the rocket and satellite communications giant. Musk allies were chosen to run NASA and the Air Force — two of SpaceX’s key customers — and one of the company’s major regulators, the Federal Communications Commission.

Elon Musk will maintain his presence in President Donald Trump’s orbit when he leaves the White House, Trump and Musk said Friday during a joint press conference to mark Musk’s last day in the administration.