Scott Kelly

U.S. Navy Capt. Scott Kelly holds the American record for the longest consecutive time in space, after a 340-day mission to the International Space…

After a year in space, Kelly says, writing a book was harder than he thought — but still, he adds, "If I write a bad sentence people are only going to get angry with me. They're not going to die."

During astronaut Scott Kelly's year in space, scientists will compare his physiology with that of his twin brother, Mark, to study the effect of prolonged space flight on the human body.

Before NASA can send a manned mission to Mars, it needs to know more about how extended time in space affects the human body. Mark and Scott Kelly are the perfect subjects for such an experiment.