On Tuesday, Scott J. Kelly is scheduled to return from the International Space Station, completing the longest stay in space for a NASA astronaut. Here are some ways to measure his mission.

Scott Kelly’s Year in Space

On Tuesday, Scott J. Kelly is scheduled to return from the International Space Station, completing the longest stay in space for a NASA astronaut. Here are some ways to measure his mission.

The Helix Nebula, also known as the "Eye of God".

The Helix Nebula, also known as the "Eye of God".

In 1054 AD, Chinese astronomers were startled by the appearance of a new star, so bright it could be seen in broad daylight for several weeks. Today, the Crab Nebula is what's left of the supernova explosion they witnessed.

In 1054 AD, Chinese astronomers were startled by the appearance of a new star, so bright it could be seen in broad daylight for several weeks. Today, the Crab Nebula is what's left of the supernova explosion they witnessed.

‘Year in Space’ Special Debuts on PBS Wednesday

'Year in Space' Special Debuts on PBS Wednesday

‘Year in Space’ Special Debuts on PBS Wednesday

Nasa astronaut Scott Kelly (pictured) is among the astronauts to have taken stunning images of Earth from space over the past year

Astronaut pictures from ISS give a unique perspective on 2015

Nasa astronaut Scott Kelly (pictured) is among the astronauts to have taken stunning images of Earth from space over the past year

(March 1, 2016) — [Breaking news update, posted at 11:34 p.m. ET] NASA astronaut Scott Kelly and Russian cosmonauts Mikhail Kornienko and Sergey Volkov have landed in the Kazakhstan desert af…

(March 1, 2016) — [Breaking news update, posted at 11:34 p.m. ET] NASA astronaut Scott Kelly and Russian cosmonauts Mikhail Kornienko and Sergey Volkov have landed in the Kazakhstan desert af…

Last year in science

Last year in science

Astronaut Scott Kelly is set to return to Earth next week after spending a record 340 days in space. He snapped hundreds of breathtaking pictures of the home planet from the International Space Station.

Astronaut Scott Kelly is set to return to Earth next week after spending a record 340 days in space. He snapped hundreds of breathtaking pictures of the home planet from the International Space Station.

He lived on the International Space Station for nearly a year. Now that Kelly is home, the real science begins.

NASA astronaut Scott Kelly returns to Earth after 340 days in space

He lived on the International Space Station for nearly a year. Now that Kelly is home, the real science begins.

Astronaut Virgil "Gus" Grissom, the second American to fly into space, shown strapped in a centrifuge during a simulated space flight, 1959. Lieutenant Col. Grissom was killed, along with fellow astronauts Roger Chaffee and Ed White, in a launch pad fire while training for the Apollo 1 mission in January 1967.

Barbra Streisand

Astronaut Virgil "Gus" Grissom, the second American to fly into space, shown strapped in a centrifuge during a simulated space flight, 1959. Lieutenant Col. Grissom was killed, along with fellow astronauts Roger Chaffee and Ed White, in a launch pad fire while training for the Apollo 1 mission in January 1967.

Cool!

Cool!

IN SPACE - DECEMBER 21: In this handout photo provided by NASA, NASA astronaut Scott Kelly snaps a quick selfie during a spacewalk on December 21, 2015 in space. NASA astronauts Scott Kelly and Tim Kopra released brake handles on crew equipment carts on either side of the space stations mobile transporter rail car so it could be latched in place ahead of Wednesdays docking of a Russian cargo resupply spacecraft. Kelly and Kopra also tackled several get-ahead tasks during their three hour…

IN SPACE - DECEMBER 21: In this handout photo provided by NASA, NASA astronaut Scott Kelly snaps a quick selfie during a spacewalk on December 21, 2015 in space. NASA astronauts Scott Kelly and Tim Kopra released brake handles on crew equipment carts on either side of the space stations mobile transporter rail car so it could be latched in place ahead of Wednesdays docking of a Russian cargo resupply spacecraft. Kelly and Kopra also tackled several get-ahead tasks during their three hour…

Mind-blowingly beautiful photos from Astronaut Scott Kelly's record-breaking year in space will make you fall in love with Earth all over again

Mind-blowingly beautiful photos from Astronaut Scott Kelly's record-breaking year in space will make you fall in love with Earth all over again

August 10, 2007 – The Space Shuttle Endeavour performs a “rendezvous pitch maneuver” prior docking with the International Space Station. (NASA)

August 10, 2007 – The Space Shuttle Endeavour performs a “rendezvous pitch maneuver” prior docking with the International Space Station. (NASA)

Cosmonaut Elena Serova.  Elena will become only the fourth female Russian Cosmonaut to fly in space and the first to live and work aboard the International Space Station when she launches on Soyuz TMA-14M on Thursday, Sept. 25, 2014 at 4:25 p.m. EDT.

Cosmonaut Elena Serova. Elena will become only the fourth female Russian Cosmonaut to fly in space and the first to live and work aboard the International Space Station when she launches on Soyuz TMA-14M on Thursday, Sept. 25, 2014 at 4:25 p.m. EDT.

Pluto's Moons Out of Whack: Much of the galaxy is orderly, with precise orbits and revolutions and rotations, an interplanetary precision moving everything around the sun. But not so with Pluto and its moons. Two moons, Nix and Hydra, wobble unpredictably as they wheel around the tiny dwarf planet. | Image: NASA

Pluto's Moons Out of Whack: Much of the galaxy is orderly, with precise orbits and revolutions and rotations, an interplanetary precision moving everything around the sun. But not so with Pluto and its moons. Two moons, Nix and Hydra, wobble unpredictably as they wheel around the tiny dwarf planet. | Image: NASA

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