In the late 1940s, the notion of space travel lived squarely in the realm of science fiction. But a young Army doctor named John Paul Stapp saw no limit to how far mankind could go—he had his eyes set on the heavens. By the 1950s and early '60s, a small band of high-altitude pioneers exposed themselves to the extreme forces of space, long before NASA's acclaimed Mercury 7 would make headlines. With a fraction of NASA’s budget and none of its renown, Stapp's Project Excelsior would send Captain Joseph Kittinger to a record-breaking 102,800 feet above Earth, lifted not by rocket, but by balloon. Though largely forgotten, this group of daring explorers would be the first to venture into the frozen vacuum on the edge of our world, testing the very limits of human physiology and ingenuity in this deadly realm.