NASA astronaut Scott Kelly (left) and Russian cosmonaut Mikhail Kornienko (right) marked their 300th consecutive day aboard the International Space Station on Jan. 21, 2016. The pair will spend a total of 340 days in space on their one-year mission as researchers hope to better understand how the human body reacts and adapts to long-duration spaceflight. This knowledge is critical as NASA looks toward human journeys deeper into the solar system, including to and from Mars, which could last 500 days or longer.