Barely a year ago, the moon was a distraction to Elon Musk, the billionaire chief executive of SpaceX then fixated on his ludicrously ambitious project to build a self-sustaining city on Mars within 20 years. Why bother returning to the orbiting chunk of rock humanity conquered half a century ago, he reasoned, when the greater prize of the red planet lay tantalizingly in reach for his company's mighty Starship rockets?