Walker, who flew P-38 Lightnings during World War II, became a test pilot in the early ’50s and gained experience in a variety of research aircraft, including the Bell X-1, X-5 and Douglas X-3, which he said was the worst plane he ever flew. But he made his name flying North American Aviation’s X-15.

Walker made his first X-15 flight in 1960 and was completely surprised by the plane’s power, hollering, “Oh my God!” as the afterburners kicked in (and eliciting a joking, “Yes? You called?” from a ground controller). But he would go on to make 24 flights in the X-15, including the memorable July 19 ascent, known as Flight 90.

Breaking the 100-kilometer barrier also meant penetrating the threshold of space, so the flight qualified Walker as an astronaut. When he repeated the feat a month later, he became the first person to enter space twice.

Walker also recorded the fastest speed ever reached in an X-15: On June 27, 1962, he hit 4,104 mph, or Mach 5.92.