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Scott Kelly

Alfred A. Knopf, Inc.

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 Station in 2015 and 2016.

Since returning to Earth, he's written a number of books about his experiences, including "Endurance: A Year in Space, A Lifetime of Discovery," and a children's book, "My Journey to the Stars."

As part of NASA's twins study, astronaut Scott Kelly spent a year in space while his twin brother, astronaut Mark Kelly, stayed on Earth. That year on the space station makes Scott Kelly the American record holder for consecutive days in space. To get through that year, he had a routine.

Updated at 4 p.m. ET

A Russian rocket has carried a Russian cosmonaut and an American astronaut to the International Space Station, where they will live for a full year, twice as long as people usually stay.

No American has remained in space longer than 215 days. Only a few people have ever gone on space trips lasting a year or more — the longest was 437 days — and they're all Russian cosmonauts. The last year-plus stay in space occurred nearly two decades ago.

This month, NASA revealed new details of the plan to send humans to Mars by 2030. It's an elaborate and expensive mission, involving a giant deep-space rocket, and roping an asteroid into the moon's orbit to use as a stepping stone to Mars.

But there are still some serious questions about a manned expedition to Mars. Namely, is it safe? That's where astronauts Scott and Mark Kelly come in. The Kelly brothers are identical twins, and the only siblings ever to both fly in space.