About 25 years ago, Adeo Ressi, a former college roommate of Elon Musk¡¯s, begged the tech entrepreneur not to start a rocket company.

Musk had just reaped millions of dollars from the sale of PayPal, which he helped create, to eBay. He and Ressi were looking at ways to send terrestrial plant life to Mars. They had a $50 million budget, but soon found it was not enough to cover the cost of a rocket. So Musk told his friend that he was going to build his own.

Ressi said he had organized a panel of space experts one day in a conference room of a hotel in Santa Monica, California, and brought Musk in to hear why the idea was a fool¡¯s errand. Private spaceflight was expensive, and the economics made little sense, the dozen or so people there said. But Musk did not heed them.