In this Dec. 21, 2015 photo, Expedition 46 Commander Scott Kelly participates in a spacewalk outside the International Space Station in which he and Flight Engineer Tim Kopra, not pictured, moved the station's mobile transporter rail car ahead of the docking of a Russian cargo supply spacecraft. (NASA via AP)

Space superman Scott Kelly's yearlong mission is one for the NASA history books.

The NASA astronaut has been living at the International Space Station since last March. By the time he lands in Kazakhstan next week, he will have spent 340 consecutive days in space, a U.S. record.

Kelly will have completed 5,440 laps around the planet. Logged 143.8 million miles. Welcomed nine visiting spacecraft. Shared the place with 14 other humans, including his roommate for the long haul, Russian cosmonaut Mikhail Kornienko, also returning to Earth.

He drank nearly 200 gallons of recycled urine and sweat, collected from himself and everyone else on board.

In this Saturday, March 21, 2015 photo, Expedition 43 NASA Astronaut Scott Kelly goes through spin chair training during a media day in Baikonur, Kazakhstan. Kelly, and Russian Cosmonauts Gennady Padalka, and Mikhail Kornienko of the Russian Federal Space Agency (Roscosmos) headed to the International Space Station in a Soyuz TMA-16M spacecraft from the Baikonur Cosmodrome on March 28, 2015. As the one-year crew, Kelly and Kornienko are scheduled to return to Earth in March 2016. (Bill Ingalls/NASA via AP)

He snapped hundreds of breathtaking pictures of the home planet and posted them on Instagram and Twitter as StationCDRKelly, not to be confused with ShuttleCDRKelly, retired astronaut Mark Kelly, his identical twin. The brothers have taken part in an unprecedented twins study over the past year.

The 52-year-olds say they're proud to have served as guinea pigs for future Mars expeditions. For NASA, getting astronauts to Mars in 20 years is what this yearlong mission is all about.

  • In this Saturday, March 28, 2015 photo, a Soyuz TMA-16M spacecraft launches to the International Space Station carrying Expedition 43 NASA Astronaut Scott Kelly and Russian Cosmonauts Mikhail Kornienko and Gennady Padalka of the Russian Federal Space Agency (Roscosmos) from the Baikonur, Kazakhstan cosmodrome. As the one-year crew, Kelly and Kornienko are scheduled to return to Earth in March 2016. Photo Credit (NASA/Bill Ingalls)

  • In this July 12, 2015 photo, Astronaut Scott Kelly takes a photo of himself inside the Cupola, a special module of the International Space Station which provides a 360-degree viewing of the Earth and the station. (Scott Kelly/NASA via AP)

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