Scott Kelly returned from the International Space Station two inches taller than his identical twin, Mark.

Temporary tallness is one of the many strange effects of spending almost a year aboard the International Space Station. It’s fairly simple to explain: Without normal gravity pushing down on the body, the spine expands, then quickly returns to normal back on Earth.

Researchers are trying to understand what happens to the human body during lengthy missions to prepare for an eventual — and much longer — journey to Mars and back. Now, for the first time ever, scientists are comparing Scott’s year in space and Mark’s year on earth in a “twin study.”

Twin studies enable scientists to better understand the relative importance of genetics or the environment in a study’s results.

As identical twins, the two men share essentially the same genetics. Both have spent significant time in space, with Mark logging 54 days aboard the Space Shuttle. Mark does note a few differences, however, when the two were in high school:

“I think I took the harder classes, because, you know, [Scott] had a hard time with those,” he joked in an interview with NASA.

A collaborative effort

Astronaut Scott Kelly and cosmonaut Mikhail Kornienko posing, with Kelly holding a flower and Kornienko a sign (NASA)
During a long spaceflight, astronaut Scott Kelly and cosmonaut Mikhail Kornienko pose with one of their experiments: space-grown zinnias. (NASA)

Throughout his year in space, Scott Kelly worked closely with Russian cosmonaut Mikhail Kornienko on experiments to prepare humans for long trips in space. NASA, the Russian space program Roscosmos, and other international partners are studying changes in astronauts’ eyesight, gut microbes, cells and psychology, among other things.

The twins and Kornienko have released a trove of health data to researchers, which will be analyzed and published over the next several years.

What’s next? “Going to Mars is doable,” said Scott, although he won’t be the one making the journey. He announced his retirement from NASA as of April. “We’re close enough that if we make the choice, I think we can do it.”

Science isn’t all Scott Kelly did in his year in space. In between hundreds of other experiments, he was able to take some amazing pictures — and collude with his brother to sneak a different sort of spacesuit onto the station!