It is no secret that Navy SEALs, the special operations force of the US Navy, are constantly striving to out-perform themselves and each other, but how far can they go? In 1984, one of them went above and beyond his teammates and made history.

"At the time, NASA was taking astronaut candidates who were not just pilots," said Capt. William Shepherd, retired SEAL, and the first commander of the International Space Station. "There were candidates made up of doctors, engineers and scientists, and I looked at that and said, 'You know, I've spent a lot of time in the water in my SCUBA gear, and that's an awful lot like being in a space suit, so I think I'll just apply and see what happens.'"

Not long after, Shepherd learned he would become a member of the NASA Astronaut Corps, making him not only the first military non-aviator, but also the first Navy SEAL to go through astronaut training in US history.

After four years of training, Shepherd embarked on the space shuttle mission STS-27 (Space Transformation System 27), and launched into space for his first time December 2, 1988 from the John F. Kennedy Space Center, Cape Canaveral, Florida.

"We are now at a crossroads, deciding whether we are bound to inhabit only the Earth, or if humans are to live and work far from the home planet," said Shepherd in an interview regarding the fifth anniversary of continuous life aboard the International Space Station. "Let us continue now with new explorations which are more expansive and more bold; voyages which will define us as a space-faring civilization."

Navy Lt. Cmdr. Chris Cassidy in his space suit during zero gravity training at the NASA Neutral Buoyancy Lab in Houston, March 24, 2009. US Navy

Shepherd's path to becoming an astronaut in service to the country started with Underwater Demolition Team Eleven, then SEAL Teams One and Two, and Special Boat Unit Twenty, all operating in the Pacific, Atlantic, and European theaters.

Although Shepherd was standing in the ranks among the most highly trained warriors in the world, he looked toward the stars to achieve more, he wasn't the only one. Capt. (SEAL) Chris Cassidy, a current astronaut, spent more than 10 years in the SEAL teams and was directly influenced by the previous achievements of Shepherd.

"In my experience with the SEAL teams and with going through BUD/S, it's given me the confidence to know I can accomplish anything that I want," said Cassidy in an interview with US Navy SEAL & SWCC Page. "If you look at SEALs after their life in the teams, you'll find people in all different sectors of industry doing all types of things. I personally always had an interest in astronauts, and I followed Capt. Shepherd's career and was inspired by him to be an astronaut."

In 2004, Cassidy was selected for NASA's Astronaut Candidate Class and joined a group of fellow explorers including pilots and engineers. Shortly after selection, he began intensive training that included land survival, T-38 jet ground and flight training, Shuttle orbiter systems training, space station systems training, science and engineering briefings and orientation tours at all NASA centers, including the Kennedy Space Center and Marshall Space Flight Center, Huntsville, Alabama.

Currently, Cassidy is in command of the International Space Station on "Expedition 63." The current mission he leads is conducting research investigations focused on biology, earth science, human research, physical sciences and technology development, as well as providing the foundation for continuing human spaceflight beyond low-earth orbit to the Moon and Mars which is central to future space exploration as part of NASA's Artemis program.

Navy Lt. Cmdr. Chris Cassidy is lowered into the Neutral Buoyancy Lab for a mission training session in Houston, March 24, 2009. US Navy

The Artemis program is an ongoing US government-funded crewed spaceflight program with the goal of landing "the first woman and the next man" on the Moon by 2024, and it is likely that a US astronaut currently serving in the program will be the next American to step on the surface of the Moon.

It is possible that astronaut could be Lt. (SEAL) Jonny Kim.

In 2002, Kim decided to leave his hometown of Santa Monica, California to enlist in the Navy and join the ranks of Naval Special Warfare operators.

"I didn't like the person I was growing up to become. I needed to find myself and my identity," said Kim in an interview with former SEAL, Jocko Willink. "And for me, getting out of my comfort zone, getting away from the people I grew up with, and finding adventure, that was my odyssey, and it was the best decision I ever made."

After completing Basic Underwater Demolition/SEAL (BUD/S), Kim was assigned as a Special Warfare Operator to SEAL Team Three Charlie Platoon and served as a Special Operations Combat Medic, sniper, navigator and point man on more than 100 combat operations spanning two deployments to the Middle East including Ramadi and Sadr City, Iraq.

His experiences as a medic taught him about teamwork, humility and service. Upon returning home, he decided to challenge himself yet again and applied for a commissioning program that put him on the path to become a medical doctor. Kim's application was accepted and he began his residency to Harvard Medical School, Boston, Massachusetts.

Jonny Kim, NASA astronaut candidate, readies for helicopter water survival training at NASA Johnson Space Center's Neutral Buoyancy Laboratory in Houston. Josh Valcarcel/NASA

In 2017, Kim was a resident physician in emergency medicine with Partners Healthcare at Massachusetts General Hospital, and Brigham and Women's Hospital in Boston. He finally met his goal of becoming a doctor, but he didn't stop there. That same year, he applied to become an astronaut and was accepted, joining NASA's team on the Artemis program.

"I was told that with the right attitude, and with enough hard work, if you get up after every time you fail, you can amount to something and you can do positive work. You can leave a positive mark for our world, and that's what I aim to do," said Kim.

Kim's unwavering perseverance led him to be the outstanding American that many call a hero, and he encourages many others to follow in his path of greatness as well.

"Don't let that hunger for the unknown go away," said Kim. "That curiosity is so important, so you should maintain that passion for what you do. Never in a million years would I have thought I could have been an astronaut candidate. I didn't have the confidence from my childhood, but dreams are possible and all good things in life are hard to get, so persevere and don't give up!"

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How the first Navy SEAL ended up in space - Business Insider - Business Insider