Watch NASA astronaut Scott Kelly return to Earth after a year in space

Where has the year gone? On Tuesday afternoon, NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko will bid the International Space Station farewell after spending 340 days in space – the longest such continuous mission ever undertaken.

Scientists hope that their unprecedented stay will offer an as-yet unparalleled window into how human beings’ health will fare in the future, if more people head into space – and for longer periods of time.

Kelly plays a special role in this experiment: His identical twin brother, fellow NASA astronaut Mark Kelly, has spent the past year with his feet planted firmly on Earth. Since the brothers have the exact same DNA, the twin astronauts are giving agency scientists a unique opportunity to examine how living in space – including a prolonged stay in microgravity and increased exposure to radiation – affects the health of the human body.

To get a baseline on the astronauts’ health, scientists have been collecting biological samples from both brothers for a full year before Scott Kelly arrived at the space station, and have continued to do so during his year in space. Researchers also expect to continue monitoring both brothers for the next year and beyond.

President Obama has set a goal of getting humans to Mars by the mid-2030s – which would require humans to spend many months in space. The set of 10 different experiments conducted on the brothers will offer scientists a totally new glimpse of the impacts that journey might have.

This twin study might have been out of this world, but Scott Kelly is now headed back to Earth. The astronaut is slated to bid the space station farewell sometime after 1:15 p.m. Pacific when he and Kornienko are scheduled to enter a Soyuz spacecraft and close the hatch, and then undock at 5:05 p.m. But the live coverage is ongoing – you can tune into NASA’s livestream, above. The astronauts are scheduled to land shortly before 8:27 p.m. in Kazakhstan.