Note: Mark Kelly's excellent mustache does not make him the evil twin.
When NASA made a decision to study identical twin astronauts - one remaining on Earth while the other orbited high above for almost one year, starting in March 2015 - scientists were not sure what they would find.
But in space, Scott Kelly's telomeres got longer. His twin brother, fellow astronaut Mark Kelly, stayed on the ground. The idea being that Mark, down on Earth, would provide a valuable control subject to compare with Scott as he spent a year in orbit. Throughout the flight, Scott self-administered sonograms of his carotid and brachial arteries, and he regularly monitored his blood pressure and collected urine and blood samples.
His telomeres - structures that protect the ends of chromosomes, much like the plastic caps on the ends of shoelaces, and which erode over time as part of the natural aging process - lengthened in space.
Months later, he still showed a slightly elevated number of cells with shortened telomeres, possibly an effect of radiation exposure.
"That's good news", added Gronostajski, who had no role in the study.
Researchers observed notable changes in chromosomes, cognitive function and eyesight, all of which were previously observed on other spaceflights.
"They give us a really in-depth view of cellular, molecular and physiological changes that can help us learn what is in the range of what a human can endure".
"Virtually all of those return to normal by six months".
"In the last six months of the mission, there were six times more changes in gene expression than in the first half of the mission", said Christopher Mason with Weill Cornell Medicine, who led one of the investigations.
"The overall Twins Study demonstrated the resilience and robustness of how a human body can adapt to a multitude of changes induced by the spaceflight environment, such as microgravity, radiation, circadian disruption, elevated CO2, isolation from friends and family and dietary limitations", said Rana.
Epigenetic changes involve chemical "tweaks" to DNA that can influence gene activity, but the changes don't affect the underlying genetic code itself.
The largest difference occurred nine months into the space mission when 79 percent of Scott's DNA was methylated, compared with 83 percent of Mark's, according to the study.
"It was encouraging to see that there was no massive disruption of the epigenome in either Mark or Scott", says Rizzardi.
The long-term health effects of long duration spaceflight are yet to be determined, but the TWINS Study represents a landmark step in humankind's journey to the moon, Mars and beyond...and to making science fiction science fact.
"To our knowledge, this team of teams has conducted a study unprecedented in its scope across levels of human biology: from molecular analyses of human cells and the microbiome to human physiology to cognition", said Craig Kundrot, director, Space Life and Physical Sciences Research and Application Division at NASA Headquarters. They found that the ratio between two particularly prominent kinds of gut bacteria dramatically shifted during spaceflight, but returned to normal once he was back on Earth.
The shape of Scott's eyeballs changed in weightlessness, leading to the types of vision problems that have been found among male astronauts (but not so much among female astronauts).
Some of the researchers caution the study, by its nature, is limited in scope.
When astronaut Scott Kelly landed in the frigid Kazakhstan plains on March 2, 2016, a team of responders pulled Kelly and two Russian cosmonauts from the charred capsule and carried them to chairs, set out in the crisp morning air. Scott's ratio between these two phyla changed while he was in space, with the ratio of these two phyla increasing about five-fold relative to his baseline.
Those are just some of the lingering question that researchers will hope to answer in the future.
What is unknown is exactly why space travel impacts age and health.
Other co-investigators working with Rana included Tomas Vaisar and Andy Hoofnagle at the University of Washington; Immaculata De Vivo at Harvard School of Public Health; and Stuart Lee, Brandon Macias and Mike Stenger at the Johnson Space Center. The study's results showed that Scott Kelly's body responded appropriately to the vaccine, enabling NASA to have more confidence in immune system response on long-duration missions.