Jonny Kim, age 35, graduated in January as a full member of the NASA astronaut corps. He may one day join a mission to the moon or Mars.

Before he was an astronaut, though, Kim served as a Navy SEAL who went on 100 combat missions, racked up service medals, earned a math degree, became a medical doctor, and more.

Kim told Business Insider one of the most important lessons he was ever taught came from one of his greatest mistakes, which was during the Iraq War and involved a close friend getting shot in the face.

Kim said he learned that "ego is probably one of the biggest poisons" in society and that "it's toxic to any environment."

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The path to success is sometimes riddled with hardship. For combat veterans especially, those moments can be exceedingly cutting and difficult to talk about with those who've never served.

But brand-new NASA astronaut Jonny Kim, age 35 — who in his previous life earned a math degree, became a medical doctor, and participated in 100 combat missions as a Navy SEAL (and who makes US senators feel "horribly, horribly inadequate") — recently opened up in an interview with Business Insider about what he said was perhaps his greatest mistake.

Kim is a first-generation Korean-American and, in the not-too-distant future, be asked by NASA to risk his life rocketing to the moon or Mars.

If he makes either or perhaps even both trips, he will take with him that and other lessons of his past.

Kim says his 'greatest mistake' came after a friend in the service was shot in the face

Kim says it's important "to learn a lot every time you fail" — a tough dynamic to embrace.

"If I could talk to my younger self, I would just say that the path to great things is filled with a lot of stumbles, suffering, and challenges along the way," he told Business Insider. "But if you have the right attitude and know that hard times will pass — and you get up each time — you will reach your destination."

Kim says his greatest mistake happened during the Iraq War in 2006. Leading up to that moment was a preceding one, during which was serving as a combat medic who was "seriously outranked" by a physician's assistant (PA) following a mortar attack. After Kim struggled to insert an intravenous needle to stabilize a wounded soldier, he said, the PA was "giving him a hard time."

"I was professionally embarrassed. It affected me and I wanted to do better, but I was also intimidated by this guy because he was older than me, he outranked me, and had a lot more experience," Kim said. "He kind of was making me feel small."

Not too long after that, he said, his friend and sniper Ryan Job was shot in the face in the line of duty. (Job survived the attack yet died several years later during facial reconstruction surgery.) Kim said he managed to stabilize Job, but disagreed with the PA about what to do next; Kim wanted to get him to an operating room right away, while the PA wanted to insert a breathing tube up his nose — what he said is a very painful procedure — during a stopover en route to a hospital.

"That's something you would never do for someone with suspected maxillary fractures, facial fractures, which Ryan definitely did have from being shot in the face," Kim said. "I remember getting in a really heated argument, trying to do my best to stand up for my friend, because I know that this procedure that this physician's assistant wanted to do would severely hurt if not kill my friend."

At that point, Kim said "nearby Army soldiers" asked him to "clear and safe" his weapons outside, per a strict Army regulation, at which time he reluctantly left Job's side.

When he returned, the PA had inserted the device into his friend.

"I feel that that is one of the biggest failures I've ever had: It was protecting my friend. I knew what the right thing was. I don't think I fought hard enough to protect him," Kim said. "I knew what the right thing was to do and I was intimidated by this senior figure, but it wasn't a good excuse for me."

Kim knows he stood up for his friend, and hard, but says he wonders today "if there was something more he could have done" in terms of taking a more extreme measure, like "refusing to go outside to clear his weapon or threatening to report the PA to his superiors."

The lesson he learned, he said: "Ego is probably one of the biggest poisons we can have — it's toxic to any environment."

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