Elon Musk's latest pay package has a distinctly SpaceX twist: To unlock all of it, he'll have to help build a city on Mars. In SpaceX's long-awaited S-1 filing on Wednesday, the company disclosed that its board approved a grant of 1 billion performance-based restricted shares for Musk as part of a compensation plan tied not only to valuation milestones, but also to one of Musk's most ambitious goals yet: establishing "a permanent human colony on Mars with at least one million inhabitants." The