After living for nearly a year aboard the International Space Station, NASA astronaut Scott Kelly is two inches taller than his identical twin brother Mark.

One of the main goals of his groundbreaking mission is to study how well humans can endure — mind, body and spirit — on a long-duration spaceflight.

Kelly, who has spent more time in space than any other American astronaut, reports that overall he "feels pretty good" and now begins what may be a year-long project to monitor his health. One unique advantage he provides to NASA's doctors is his identical twin brother, Mark, a former NASA astronaut who spent last year with his feet planted on terra firma.

Comparing the twins will help researchers spot any genetic changes that might have occurred in Scott in space.

See images of Scott Kelly returning from space:



18 PHOTOS Astronaut Scott Kelly returns to Earth See Gallery NASA's Scott Kelly grew 2 inches: The body after a year in space Members of NASA support team help International Space Station (ISS) crew member Scott Kelly of the U.S. to get off a helicopter on arrival from the landing site at the airport of the town of Dzhezkazgan, Kazakhstan, on March 2, 2016. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth on March 2 after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars. AFP PHOTO / POOL / KIRILL KUDRYAVTSEV / AFP / POOL / KIRILL KUDRYAVTSEV (Photo credit should read KIRILL KUDRYAVTSEV/AFP/Getty Images) Expedition 46 Commander Scott Kelly of NASA rests in a chair outside of the Soyuz TMA-18M spacecraft just minutes after he and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos landed in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) International Space Station (ISS) crew member Scott Kelly of the U.S. shows a victory sign after landing near the town of Dzhezkazgan, Kazakhstan, on March 2, 2016. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth on March 2 after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars. AFP PHOTO / POOL / KIRILL KUDRYAVTSEV / AFP / POOL / KIRILL KUDRYAVTSEV (Photo credit should read KIRILL KUDRYAVTSEV/AFP/Getty Images) Russian cosmonauts Mikhail Kornienko, left, Sergey Volkov of Roscosmos, center, and Expedition 46 Commander Scott Kelly of NASA, rest in chairs outside of the Soyuz TMA-18M spacecraft just minutes after they landed in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) Russian cosmonaut Mikhail Kornienko of Roscosmos is carried into a medical tent after he and Expedition 46 Commander Scott Kelly of NASA and Russian cosmonaut Sergey Volkov landed in their Soyuz TMA-18M spacecraft in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) Expedition 46 Commander Scott Kelly of NASA rests in a chair outside of the Soyuz TMA-18M spacecraft just minutes after he and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos landed in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) Russian cosmonaut Sergey Volkov of Roscosmos is carried into a medical tent after he and Expedition 46 Commander Scott Kelly of NASA and cosmonaut Mikhail Kornienko landed in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) International Space Station (ISS) crew member Scott Kelly of the U.S. reacts after landing near the town of Dzhezkazgan, Kazakhstan, on March 2, 2016. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth on March 2 after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars. AFP PHOTO / POOL / KIRILL KUDRYAVTSEV / AFP / POOL / KIRILL KUDRYAVTSEV (Photo credit should read KIRILL KUDRYAVTSEV/AFP/Getty Images) Russian cosmonaut Mikhail Kornienko of Roscosmos rests in a chair outside of the Soyuz TMA-18M spacecraft just minutes after he and Expedition 46 Commander Scott Kelly of NASA and Russian cosmonaut Sergey Volkov landed in a remote area near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) A search and rescue team works at the site of landing of the Soyuz TMA-18M space capsule near the town of Dzhezkazgan, Kazakhstan, on March 2, 2016. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth on March 2 after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars. AFP PHOTO / POOL / KIRILL KUDRYAVTSEV / AFP / POOL / KIRILL KUDRYAVTSEV (Photo credit should read KIRILL KUDRYAVTSEV/AFP/Getty Images) Russias Soyuz TMA-18M space capsule carrying the International Space Station (ISS) crew of US astronaut Scott Kelly and Russian cosmonauts Mikhail Kornienko and Sergei Volkov lands in a remote area outside the town of Dzhezkazgan, Kazakhstan, on March 2, 2016. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth on March 2 after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars. AFP PHOTO / POOL / KIRILL KUDRYAVTSEV / AFP / POOL / KIRILL KUDRYAVTSEV (Photo credit should read KIRILL KUDRYAVTSEV/AFP/Getty Images) The Soyuz TMA-18M spacecraft is seen as it lands with Expedition 46 Commander Scott Kelly of NASA and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) The Soyuz TMA-18M spacecraft is seen as it lands with Expedition 46 Commander Scott Kelly of NASA and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos near the town of Zhezkazgan, Kazakhstan on Wednesday, March 2, 2016 (Kazakh time). Kelly and Kornienko completed an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov returned after spending six months on the station. Photo Credit: (NASA/Bill Ingalls) NASA astronaut Scott Kelly hugs his crewmates goodbye on the International Space Station on March 1, 2016. Photo credit: NASA NASA astronaut Scott Kelly with his crewmates on the International Space Station on March 1, 2016. Photo credit: NASA NASA astronaut Scott Kelly hugs his crewmates goodbye on the International Space Station on March 1, 2016. Photo credit: NASA NASA astronaut Scott Kelly hugs his crewmates goodbye on the International Space Station on March 1, 2016. Photo credit: NASA NASA astronaut Scott Kelly hugs his crewmates goodbye on the International Space Station on March 1, 2016. Photo credit: NASA Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Meanwhile — here's what National Space Biomedical Research Institute and NASA tells us they'll be on the lookout for:

What a Year in Space Can Do to Your Body

BRITTLE BONES

Astronauts no longer walk to get in the spacecraft, they float so the bones in the legs, hips and spine experience a significant decrease in load bearing.

This leads to bone breakdown and a release of calcium, leaving the bone more brittle and weak.

The release of calcium can also increase the risk of kidney stone formation and bone fractures.

WEAK MUSCLES

Extended spaceflight results in less work for the legs and back: muscles can begin to weaken or atrophy, and this could lead to fall-related injuries and accidents during exploration missions.

PUFFY FACE AND LEGS

In space, blood is flowing more in the upper part of the body and a little less in the lower extremities.

While in space, astronauts often have a puffy face and the legs that are smaller in circumference.

SMALLER HEART

The heart doesn't have to work as hard up there: Over time, this could lead to a decrease in the size of the heart.

There is also a concern that space radiation may affect endothelial cells, the lining of blood vessels, which might initiate or accelerate coronary heart disease.

BALANCE ISSUES

The inner ear, which is sensitive to gravity, no longer functions correctly. Early in the mission, astronauts can experience disorientation, space motion sickness and a loss of sense of direction.

Upon return to Earth, they must readjust to Earth's gravity and can experience problems standing up, stabilizing their gaze, walking and turning.​

Photos: A Look Back at Kelly's Year in Space

INCREASE CANCER RISK

Scientists know that astronauts are exposed to higher levels of radiation in space that could potentially lead to cataracts and cancer.

​Related: Astronaut Scott Kelly's very social #YearInSpace

MESSED UP BODY CLOCK

There is no 24-hour day/light cycle in space so the astronaut's body clock has to readjust to day/light cycle after return to earth.

And it seems Scott Kelly is correct about his new height.

ASTRONAUTS GET TALLER IN SPACE

Astronauts get a bit taller in space because of the disks of the spinal column: On Earth, the disks are slightly compressed due to gravity. In space, that compression is no longer present causing the disks to expand. The result: the spine lengthens, and the astronaut is taller.

Dr. Felix Gussone is associate medical producer with NBC News; Dr. Shelly Choo is a medical fellow with NBC News