I was only five when John Glenn became the first American to orbit Earth on February 20, 1962. I don't recall watching the flight on television but I do remember the issue of National Geographic with a cover painting of Glenn and his Mercury capsule plunging back into the atmosphere in a glowing trail of flame. Inside, the article explained that Glenn’s reentry was made even more harrowing by ground controllers’ suspicion—unfounded as it turned out—that the heat shield on his Friendship 7 spacecraft was loose. The words he spoke when he regained contact with Mercury control—“That was a real fireball, boy”—instantly became one of the great heroic sound bites of my childhood.

But it took much longer for me to really understand who that larger-than-life figure really was—how much more he was than a Right Stuff test pilot, more even than a “point of the spear” in the Cold War space race with the Soviet Union. The more I've come to know about John Glenn and what he did, the more I've come to see him as the quintessential astronaut.

Glenn was a man who seemed to be driven by one overriding purpose: to utilize his abilities to the fullest extent possible and to increase what he once called “mankind’s ever broadening store of knowledge.” He welcomed the unknown and longed to explore it. Without planning it, or even realizing it, he was preparing for his Mercury mission for most of his adult life.

Glenn cut short his college career to join the Marines and fly fighters in World War II. In Korea he flew jets, going after combat missions with such fervor that his squadron mates nicknamed him the “MiGMad Marine.” Glenn was a test pilot who had just set a transcontinental speed record in late 1957 when Sputnik launched the space age, and he hungered to be part of it. Soon he was volunteering to ride a giant centrifuge to test a pilot’s abilities to control a spacecraft under the crushing acceleration of launch and reentry. The following year, when a newly formed NASA went looking for astronauts, Glenn was an obvious choice. In fact, as Mercury astronaut selection chief Charles Donlan told me when I interviewed him in 1997, “Glenn was the first guy we picked.”

As much as Glenn was frustrated not to be chosen for Mercury’s first flight—a 15-minute suborbital “hop” that his colleague Alan Shepard flew in 1961—the first orbital mission was an assignment that brought out Glenn’s abilities even more. He had once thought about becoming a doctor, and he was fascinated by the medical aspects of his upcoming voyage. This was a time when only two humans had been in orbit, both Soviet cosmonauts, and no one could say how Glenn would respond to being weightless for four and a half hours during his three orbits. In the aerospace medical community doubts swirled. Would his eyes change shape or rapidly flick back and forth in a condition called nystagmus? Would the state of continual free fall play havoc with his inner ear, causing incapacitating motion sickness?

Those fears proved groundless. When Glenn finally reached orbit he was nothing less than a man completely at home in that alien environment. Not only did he not feel sick, he found zero g to be a thoroughly pleasant sensation. Before the flight he’d lobbied the astronauts’ boss, Bob Gilruth, to let him bring a camera along, to capture the sights no American had ever seen. He’d even bought the camera himself at a Cape Canaveral drugstore. In orbit he aimed it through Friendship 7’s single, trapezoidal window to photograph deserts and mountains in Africa, the cloud-flecked ocean and nearly the entire Florida peninsula in one glance, all set under a thin, glowing blue ribbon of atmosphere separating Earth from the blackness of space. As the capsule sped eastward at 17,500 miles per hour, he was soon shielding his eyes from the sun’s unfiltered, arc-lamp brilliance, then marveling at the spectacle of an orbital sunset as a rainbow band of color blossomed along the curved horizon. Over Earth’s night side, in the light of a full moon, he gazed down at a huge storm sprawled across the Pacific, with flashes of lightning going off like firecrackers out to the horizon.

And then there were the “fireflies”—strange, glowing, yellow-green particles that surrounded the capsule as it flew into orbital dawn. Not until a few months later, when astronaut Scott Carpenter made his own orbital mission, would everyone realize they were ice droplets created by water venting from the spacecraft. As Glenn studied them through the capsule’s window, they were a mystery. When he described them to ground controllers his voice was full of curiosity and excitement—the voice of an explorer in a new and endless frontier.

It’s Glenn the explorer I think about now, as much as Glenn the test pilot. I find myself wondering what it would have been like to see him walk on the moon. Glenn was such an icon that many people just assumed he had. When I was writing my book about the Apollo missions, A Man on the Moon, it seemed nearly everyone I mentioned it to asked, “Are you going to interview John Glenn?” I had to explain that Glenn never got the chance to go to the moon. He would have loved to have made the trip but in 1964 Bob Gilruth had told him that by the time the Apollo missions were flying, he’d be close to 50—and that, Gilruth said, was too old.

No one could have imagined then that more than three decades later a 77-year-old Glenn would shatter all preconceptions of age by venturing into space for a second time. I was lucky enough to witness his launch on space shuttle Discovery, watching him and his six crewmates leave the planet atop brilliant pillars of fire. And when it was over, I was able to turn to a fellow journalist and say, as I had not known to do in 1962, “John Glenn is in orbit.”