Carried aloft beneath the wing of a B-52 bomber, the X-15 was dropped at a launch altitude of thirty-five thousand feet. The pilots fired their rocket engines for just ninety seconds, climbing steeply and accelerating to hypersonic speed, rising in a parabolic arc to the edge of space. They watched the curvature of the Earth expand beneath them, the sky darken from blue to black, and the pages of their cockpit flight plans float apart in zero gravity.
