Twin Paradox...

Retired astronaut Mark Kelly (left) cracks a slight smile while posing with his identical twin brother, astronaut Scott Kelly (right). As part of NASA's Twins Study, Scott took a long trip to space, while Mark remained on Earth. Researchers then monitored how their bodies reacted to their differing environments. NASA

Topics: Astronaut, Astrophysics, Genetics, NASA, Spaceflight

Brothers compete. So in 2016, when astronaut Scott Kelly returned to Earth after spending a year in space, it must have really annoyed his identical twin brother — retired astronaut Mark Kelly — that Scott was two inches taller than when he left. However, Scott's temporary increase in height was not the only thing that changed during his trip.

As part of NASA's Twins Study, while Scott was in space, Mark went about his daily life on Earth. Over the course of the year-long mission, researchers tracked changes in both brothers' biological markers to pinpoint any variances. Because the twins share the same genetic code, researchers reasoned that any observed differences could tentatively — though not definitively — be linked to Scott's time aboard the International Space Station (ISS). This allowed them to take advantage of a unique opportunity and explore how an extended stay in space may impact the human body.

Based on their results, which were published this week in the journal Science, spaceflight can definitely trigger changes in the human body. But the vast majority of these changes disappear within just a few short months of returning to Earth.

Most notably, the researchers found that living in a microgravity environment can: damage DNA; impact the way thousands of individual genes are expressed; increase the length of telomeres (the shielding caps that protect the ends of our chromosomes); thicken artery walls; modify the microbiome; and increase inflammation — just to name a few.

"This is the dawn of human genomics in space," said Andrew Feinberg, a distinguished professor at Johns Hopkins University and one of the lead investigators for the Twins Study, in a press release. "We developed the methods for doing these types of human genomic studies, and we should be doing more research to draw conclusions about what happens to humans in space."

NASA's Twins Study: Spaceflight changes the human body, but only temporarily
Jake Parks, Astronomy