Of NASA’s 165 human spaceflights, the third was perhaps the most urgent.

Fifty years ago, a red-headed Marine colonel, John H. Glenn Jr., strapped into a tiny Mercury capsule known as Friendship 7 and hurtled into space. Glenn circled the Earth three times in just under five hours, America’s Space Age dreams looping along with him.

The Soviet Union had launched Yuri Gagarin into orbit 10 months earlier, taking a triumphant lead in the accelerating space race. In the intervening months, the United States managed to put two astronauts aloft — Alan Shepard and Virgil “Gus” Grissom — but only on 15-minute suborbital jaunts.

That made Glenn’s mission the big one, a signal that the United States had caught up to the Soviets. Scrub after scrub — 10 of them, for weather and technical issues — heightened the suspense.

But at 9:47 a.m. on Feb. 20, 1962, Glenn’s Atlas rocket finally roared to life, catapulting him aloft.

“Oh, that view is tremendous,” Glenn reported early in the flight. “I can see the booster doing a turnaround just a couple hundred yards behind me. It’s beautiful.”

An estimated 135 million people watched the televised coverage of the flight: a nation enthralled.

Glenn soon had to take manual control of the capsule. A glitch kept pushing it to the right. As he flew, Glenn was bemused by “fireflies” outside his window, twinkling flecks in the dark: Ice crystals venting off the craft, an absolute novelty.

Late in the flight, ground controllers grew worried that the capsule’s heat shield might fall off, incinerating Glenn on reentry. They instructed Glenn to keep the capsule’s retro rockets attached, in the hopes that their straps might keep the shield in place.

He did, and it held, and Glenn survived, the first American to orbit the Earth. He became an instant hero, with a ticker-tape parade on Lower Broadway.

Seven and a half years later, the ad­ven­ture begun by Glenn culminated with Neil Armstrong’s first footsteps on the moon during Apollo 11.

By then, Glenn had left NASA, frustrated by the agency’s refusal to schedule him on another flight. A decade later, Glenn learned that President John F. Kennedy had told NASA not to fly him again. Kennedy was apparently worried about losing his star astronaut.

“There’d been so much national attention on us, I suppose it would’ve been bad if something had happened to me,” Glenn said at a news conference marking the anniversary at the Kennedy Space Center on Friday.

It would be 36 years before Glenn made it back into space. In 1998, he launched aboard the space shuttle Discovery at age 77, another record. After retiring, Glenn served as a senator from Ohio from 1974 to 1999, with a run for the Democratic presidential nomination in 1984.

Glenn’s silver flight suit and the scorched Friendship 7 capsule , encased in hard plastic, are on display at the Smithsonian’s National Air and Space Museum.