On August 22, 1963, test pilot Joseph Walker throttled the rocket engine of the North American X-15 to punch through the atmosphere, reaching an apex of 354,200 feet—nearly 67 miles above Earth. This record-shattering flight crossed the Kármán line, marking the highest altitude ever achieved by a piloted, winged aircraft. Outside the envelope of aerodynamic control, Walker navigated the microgravity void using hydrogen-peroxide reaction thrusters, pioneering the attitude-control concepts that would later steer Apollo capsules and the Space Shuttle. Flight 91 demonstrated that the boundary between aeronautics and astronautics was a permeable engineering frontier rather than a hard wall, establishing foundational aerothermodynamic and human-factors data that still directly informs modern reusable spaceplane architecture.