Elon Musk’s rocket company SpaceX will make its debut on Wall Street Friday. Both institutional and retail investors are expected to gobble up the 555.6 million shares going up for sale at $135 apiece. Musk is already the world’s richest man and he could become its first trillionaire. Musk says the company is going public because it needs money to fund its ambitions of putting satellites and data centers in space.