On March 1st, the longest ISS mission is coming to an end. NASA astronaut Scott Kelly and Roscosmos cosmonaut Mikhail Kornienko will make their journey back to Earth, 340 days after they landed on the spacecraft. The year-long mission gave NASA's scientists the chance to study the physical and psychological effects of lengthy space travel on humans. Most astronauts only stay aboard the space station for six months -- scientists needed the data to plan for longer, deep space manned missions to Mars and other celestial bodies. It helped that Kelly has a twin brother on Earth who provided NASA "more bases for comparisons."