Built of titanium and a chrome-nickel alloy known as Inconel X, the X-15 was the fastest plane ever built, streaking through the lower reaches of outer space even before the first space capsules reached orbit. First tested in 1959, the X-15 proved to be a crucial testing ground for the astronauts and hardware in the Mercury, Gemini, Apollo, and even the Space Shuttle programs. Celebrated reporter Richard Tregaskis spent time with the pilots, engineers, and other key personnel involved in the project. We learn of the years of planning and design, devastating onboard explosions, exhilarating triumphs, and, above all, the personal and professional sacrifices that paved the way for the enduring legacy of the blisteringly fast X-15 rocket plane.