People use all kinds of words to describe Elon Musk, from “genius” to “megalomaniac,” from “visionary” to “erratic”—but now there’s less reason to call him “enigmatic,” thanks to Walter Isaacson’s new 688-page biography. Isaacson hung out with Musk for two years, attending meetings, witnessing meltdowns, taking Musk’s 3 a.m. phone calls. In this special “Unsung Science” episode, Isaacson describes the man behind Tesla,     SpaceX, Starlink, and the social-media site once known as Twitter.