U.S. astronaut Scott Kelly will begin a yearlong stay aboard the International Space Station in 2015—a trip that, if successful, will be the longest spaceflight ever by an American.
Accompanied by Russian cosmonaut Mikhail Kornienko, both men have logged time aboard the ISS, though their stays have usually been about half as long. According to NASA, "The goal of their yearlong expedition aboard the orbiting laboratory is to understand better how the human body reacts and adapts to the harsh environment of space."
The extended mission will prepare the space agency for potential future moon missions, trips to asteroids, and, eventually, a voyage to Mars.
While a yearlong stay would be one for the record books, the longest continuous spaceflight would still be held by cosmonaut Valery Polyakov, who spent 438 days aboard Russia's Mir space station in the 1990s.