In that time, the brother's bodies - their genes, guts, immune systems, blood and brains - were part of an elaborate, multifaceted study created to teach us how spaceflight might affect human bodies.
From his eyes to his immune system, astronaut Scott Kelly's body sometimes reacted strangely to almost a year in orbit, at least compared to his Earth-bound identical twin - but newly published research shows nothing that would cancel even longer space treks, like to Mars. Feinberg called the study "the dawn of human genomics in space".
The brothers were monitored for 25 months before, during and after Scott Kelly's mission to the space station from March 27, 2015, to March 1, 2016.
For the study, the team compared Scott Kelly while he was in space to Mark Kelly, his identical twin brother, who remained on Earth.
"There were ten teams of researchers but only one article", said Martha Vitaterna, first author of the Northwestern study.
NASA already knew some of the toll of space travel, such as bone loss that requires exercise to counter.
Studying twins provided a unique opportunity for researchers to observe physical and genetic differences brought on by space travel. Some results had been reported in February. Those tips gradually shorten as we get older, and are thought to be linked to age-related diseases including some cancers.
NASA discovered that the telomeres of the Kelly brother who was in space got longer. She can't explain it although it doesn't mean Kelly got younger. But returning to Earth will fix most issues, according to new research from NASA.
Turek and Vitaterna looked at how the composition of bacteria in Scott Kelly's gut changed over time and space.
"The Twins Study has been an important step toward understanding epigenetics and gene expression in human spaceflight", J.D. Polk, chief health and medical officer at NASA Headquarters, told CNN.
Again, most gene expression returned to normal back home, but some of the immune-related genes were hyperactive six months later.
The discovery shocked scientists at NASA's Human Research program, who had made a decision to use telomeres as a way to measure aging.
The scientists also found that the shape of Scott's eyeball changed over the course of the flight, including a thicker retinal nerve and folds in the choroid layer that surrounds the eye.
But in an interview with The Washington Post, Scott Kelly, now 55, said that after landing he suffered flulike symptoms and felt bad for many weeks, and that altered his cognitive performance.
Scott Kelly's immune system worked fine in space and a flu vaccine administered in orbit performed just as it would on Earth, the study found. NASA called a strong immune system critical for lengthy space missions to safeguard astronauts from disease-causing microbes in the spacecraft environment. The now-retired astronaut has acknowledged that it took at least six months for him to readjust fully.
Adjusting to life back on the ground was actually harder than adjusting to life in zero gravity, he said. Whereas Scott and Mark Kelly did not eat the same foods during the yearlong study, mice in previous studies ate the exact same diet. Feinberg and Rizzardi traveled for a week on the famed "Vomit Comet," a plane that simulates weightlessness, to test their protocols for overcoming the challenges of collecting, purifying, and storing blood samples aboard the space station. Only three-all Russian cosmonauts-have been aloft more than a year. Susan Bailey, one of the study's co-authors, said the study was the first time telomere length was measured in astronauts. The aim of the Twins Study is to provide NASA with data that ensures the health and safety of astronauts for decades to come. And longer missions, to the moon or Mars, will mean greater stress and radiation exposure. One of 10 principle investigators of the study is a professor at Colorado State University.
This result mirrored mouse studies the Northwestern pair conducted in the past. The AP is exclusively responsible for all content.