Mark and Scott Kelly are retired U.S. astronauts.

Two years before we were born and three decades before we each had the chance to leave our planet in a spaceship, our parents and 100 million other people heard the news: A 40-year-old Marine Corps lieutenant colonel named John Glenn had become the first American to orbit Earth.

To this day, Glenn’s journey remains as awe-inspiring as it was audacious. It was an act of patriotism and heroism in a life full of them.

The word “hero” gets thrown around a lot. But John Glenn defined it.

Consider that Glenn even agreed to take the elevator to the top of that Atlas rocket, climbed on and told the flight controllers to light the fuse.

As a NASA astronaut, Glenn had seen those Atlas rockets fail in unmanned tests over and over, in massive, catastrophic explosions. And the team at NASA was unsure it could bring him home safely without his capsule disintegrating during reentry. That was not exactly a confidence booster.

He knew the risks, and he flew anyway.

And as expected, that flight was far from a cakewalk. Once in orbit, the tiny capsule faced climate issues, forcing Glenn to balance the temperature of his spacesuit against the humidity inside the spacecraft.

During the flight, NASA flight controllers discovered that the capsule’s automated steering system was using up too much hydrogen peroxide — a component essential to controlling the capsule’s orientation and attitude during reentry. Glenn was faced with a choice no astronaut wants to make.

Astronauts spend hours in a flight simulator to prepare for these types of bad scenarios, but the technology then wasn’t nearly as advanced as it is today. With great intuition and skill, Glenn manually took control of the capsule to conserve power and successfully maneuvered Friendship 7 through low Earth orbit against the blackness of space. He circled our planet three times.

Four hours and 56 minutes after liftoff, Friendship 7 safely touched down in the Atlantic Ocean and was hoisted aboard the USS Noa. When he stepped out of the capsule and onto the safety of the ship’s deck, Glenn simply said, “It was hot in there.”

He was rewarded with a glass of iced tea, a call from President John F. Kennedy and, later, a ticker tape parade through New York.

Even in the face of national adoration, true heroes never think of themselves as such. Just look at Glenn.

When Pearl Harbor was attacked 75 years ago, Glenn — a kid from a small town in Ohio — dropped out of college to enlist in the Army Air Corps. He became a skilled aviator, flying 59 combat missions in the South Pacific.

In the Korean War, his wingman was none other than future Hall of Fame baseball player Ted Williams. Glenn’s buddies in the service called him “Magnet Ass” because he had a knack for attracting enemy antiaircraft artillery.

After retiring from NASA and the military, he found new ways to continue his distinguished career in public service. He represented his home state of Ohio in the Senate for 24 years. He was a champion of nuclear nonproliferation, writing legislation that helped slow the spread of nuclear weapons. It was a fitting cause for a man who made a name for himself riding a rocket into space during the Cold War.

And in 1998 at 77 years young, Glenn once again climbed onto a big rocket, ready to hurtle off the planet as if the hand of God had grabbed hold of him. Flying aboard the space shuttle Discovery, Glenn once again orbited our planet in the blackness of space.

We had the good fortune of getting to know Glenn as he trained for that final mission at NASA’s Johnson Space Center. He was gracious, humble and a lot of fun. For a couple of young astronauts, showing up to work next to John Glenn was like a rookie playing baseball alongside Babe Ruth or a kid getting music lessons while Mozart composed in the room next door.

After all, it was the idealism and courage of the early Mercury pioneers such as Glenn that helped pave the way for Gemini, Apollo, Skylab, the space shuttle program, the Hubble Space Telescope, the Mars rovers and the International Space Station. And as we stand on the precipice of a new era of exploration to Mars and beyond, it is possible only because of the gutsy pioneers and patriots such as Glenn who came first.