(CNN) The dangers of space exploration are all too clear. Last month, we observed the 50th anniversary of the Apollo 1 disaster, when a cabin fire during launch rehearsals claimed the lives of all three crew.

It was a dreadful start in NASA's quest to reach the moon. January 28 marked 21 years since space shuttle Challenger broke up 73 seconds into its flight, killing all seven aboard. Last week, NASA commemorated the lives of seven crew members who perished 14 years ago when space shuttle Columbia disintegrated above Earth during its re-entry.

Even on successful missions, crews face a multitude of issues in space. Astronauts aboard the International Space Station are exposed to radiation levels 10 times that of Earth, leading to increased cancer risks. Seventy-nine percent of astronauts are affected by space motion sickness. The heart deconditions; the immune system weakens; bones lose minerals and density at a rate of 1% per month. Fluids redistribute themselves around the body and away from the legs, and for older astronauts, near vision deteriorates.

It goes without saying that astronauts are made of stern stuff. Now retired, Capt. Scott Kelly and Shuttle Commander Mark Kelly are two of a select bunch to leave our planet and put everything on the line in doing so, not for the betterment of themselves but for mankind. For that reason, above all, CNN Chief Medical Correspondent Dr. Sanjay Gupta has chosen the identical twins for the CNN series "My Hero."

"It's easy to look at it in retrospect and say 'that was a really cool thing that you did,' but the reality is that they had to take a lot of chances," Gupta said. "They had to take a lot of risks. They had to say goodbye to their family and friends and with not the clear understanding that they'd ever see them again.

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