After 340 days in orbit, American astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko have returned safely to Earth.
A Russian Soyuz spacecraft carrying Kelly and Kornienko and a second Russian cosmonaut, Sergey Volkov, made a soft landing on the frozen steppes of Kazakhstan early Wednesday morning, three hours after separating from the International Space Station. Volkov is back after spending the last six months aboard the ISS, the typical mission length for space station crews.
Kelly now holds the American record for the longest continuous time in space, as he and Kornienko circled the world 5,440 times aboard the ISS, covering 144 million miles and witnessing 10,880 orbital sunrises and sunsets.
The duo's mission was designed to measure how a long-term stay in a microgravity environment impacts the human body and mind, with an eye toward planning for future multi-year missions that will be necessary to take crews to Mars or other places in the solar system.
WATCH: Video footage of return to Earth
Picking Kelly as a pioneer provided an additional unique advantage: His identical twin, Mark Kelly, himself a former astronaut, offered himself as a test subject so researchers could compare his Earth-bound physical condition to that of his brother's after nearly a year in space.
A Russian doctor continues to hold the overall record for consecutive days in space with 438, which was set in the mid-1990s.