As a former NASA astronaut who spent 520 days in space the second-longest stint and traveled more than 143 million miles through the cosmos, Capt. Scott Kelly knows a thing or two about taking the road less traveled.

The best part about being in space is that its a really, really hard thing to do, Kelly told more than 1,000 people Tuesday night, all huddled on the event center bleachers at the University of Colorado at Colorado Springs.

Thats what I want to talk to you about today, Kelly said. Its about doing the hard things, not the easy things.

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Hard things for Kelly included taking risks, such as becoming a test pilot for the Navy. Kelly admits he struggled, in school and flying planes. But that didnt stop him.

In my experience, when I was able to put all these things together, what I learned was that the sky is not the limit ... he said. Theres a trait that a lot of successful people have: that risk-taking mentality ... In certain points of your life, taking risks and seeing what you can be able to achieve is really an incredible trait that people have.

Kelly lived aboard the International Space Station as a commander for 340 days through three expeditions. His time aboard the space station set the record for the accumulated number of days spent in space, the single longest space mission by an American astronaut.

In this Oct. 22, 2015 photo, Expedition 45 Commander Scott Kelly tries on his spacesuit inside the U.S. Quest airlock of the International Space Station. (NASA via AP)

A unique aspect to his career was that his identical twin brother, Mark Kelly, was also a NASA astronaut. The Kelly brothers are the only twin astronauts in NASA history.

In 2015, NASA studied them to compare the effects on the human physiological, molecular and behavioral lenses. Scott spent a year at the ISS while Mark worked at ground control. When Scott returned, his body had undergone several genetic changes, most notably to his chromosomes. Researchers found the ends of Scotts chromosomes, called the telomeres, lengthened instead of shortening as they had expected.

It was a high-risk mission, Scott said, but it helped kick-start genetic testing on people during space travel, further propelling mankind into space studies. Scott said it was the most important thing hes done in his life.

Of course I learned a lot about empathy and being a good human on this planet, he said.

When you look out at planet Earth from space, you dont see political borders like you do on maps and globes of the Earth. You see a planet. You know that there people down there, and were all in this thing together called humanity.

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Former NASA astronaut Scott Kelly tells crowd at UCCS to 'do the hard things' - Colorado Springs Gazette