Last month, American Astronaut Scott Kelly was launched into space, along with Russia's Mikhail Kornienko. Together, they will spend a year on the International Space Station, completing the longest expedition there ever.

But the expedition isn't just about what will happen in space. It's also about what's happening right here on planet Earth.

That's because Scott Kelly has an identical twin brother named Mark Kelly, who's also an American astronaut, and whose body will be a basis for comparison throughout the next year, and for years after. 

Mark Kelly joins us from member station KUAZ in Tucson, Arizona to share his insights on what it's like to be an astronaut guinea pig.

In addition to having completed four missions in space, Mark is the author of a new children's book about space traveling twins called "Astrotwins."