The Astronaut Who Agreed to Leave Earth for 342 Days

On Scott Kelly's return, he will be compared to his twin brother to study what space does to the body.
1:51 | 03/27/15

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Transcript for The Astronaut Who Agreed to Leave Earth for 342 Days
Phillips' Finally tonight here, our person of the week. If you were told you wouldn't see your family for almost an entire year, and you would be floating in space for 342 days before coming back. You may say it's not for me. Tonight, the astronaut who said yes and why. Maki making history. Our person of the week. Liftoff. A year in space starts right now. Reporter: And onboard, an American astronaut. He's the one closest to us right there, walking to the spacecraft. And you might recognize his face. His name is commander Scott Kelly. His twin brother, mark, a former astronaut himself. Mark Kelly is married to gabbie giffords. Space runs in the family. But today, Scott Kelly agreed to leave his loved ones, his girfriend amiko, and his family behind for nearly an entire year. I was just this puddle of tears. Reporter: Amika remembers the last takeoff. Side by side, it is nearly impossible to tell the brothers apart. Mark's moustache is the giveaway. And he's six minutes older. Both now about to be part of an extraordinary project. Nasa will now study them with nearly identical genetic makeup to show what a year in space does to one brother and not the other. What space does to the human body. The idea being, one day, missions to Mars might take even longer than a year. What will that do to the body? Tweeting right before liftoff, enjoying a moment before my year in space. Already admitting what he'll miss. The food, not the same, and family. You still miss the people that are important to you. That's what I'll miss the most. And so we choose Scott Kelly, beginning his yearlong voyage tonight. Thank you for joining us. I'll see you later tonight on "20/20."

This transcript has been automatically generated and may not be 100% accurate.

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