The results offer important new insight as scientists look toward a long-term mission to Mars.
Ten groups of researchers are looking at a wide variety of information about the Kelly twins' health, including immune response, bone formation, their gut microbiome and how DNA might be affected by living in space.
On a genomic level, they are trying to determine whether a so-called "space gene" activated while Scott was in space. Through genome sequencing of DNA and RNA from the twins' white blood cells, the researchers found that Scott and Mark have hundreds of mutations unique to each man, with more than 200,000 RNA molecules that presented differently.
Chemical modifications to Scott's DNA decreased while he was in space, but they returned to normal once he was back on Earth. Mark's also shifted around the midpoint of the study but were normal at the end. The researchers believe that this shows how sensitive genes are to changing environments, in space or on Earth.
A focus on telomeres, the repetitive sequences at the end of a chromosome to protect it from deteriorating or fusing with other chromosomes, in the twins' white blood cells revealed a surprise: Typically, telomeres decrease in length as someone gets older. But Scott's increased while he was in space. When he returned to Earth, they shortened again.
For now, the researchers believe the change could be linked to an increase in exercise and decrease in calorie intake while on the station.
They also looked at telomerase, which repairs and lengthens the telomeres, for both men. This response increased in both Scott and Mark in November 2015, possibly related to a "significant, stressful family event" happening around that time.
Scott experienced declining bone formation, but levels of a healing hormone that helps with bone and muscle health increased, most likely due to all of the exercises astronauts perform in space every day to combat bone and muscle loss. His levels of the stress hormone cortisol remained normal, but he had a spike in inflammation soon after landing on Earth -- most likely because of the stress of re-entering the atmosphere.
Though Scott and Mark have differences in the bacteria in their digestive systems, which was to be expected because they were living in different environments and eating different food, two of the main bacterial groups switched dominant positions in Scott's microbiome between his time in space and on Earth.
The stressors of space
When it comes to a mission to Mars, the first actions astronauts take on the surface of the Red Planet could be the most crucial. But after spending six months traveling through deep space, they might have difficulty performing tasks quickly and efficiently, even if those actions would be vital to life support.
To learn more, researchers put Scott and Kornienko through a series of tests in March, after they returned to Earth from their year in space.
Their capsule landed upright on dry land, meaning they had to maneuver themselves halfway out of the capsule before being helped out. Scott is convinced that leaving the capsule would be easier on Mars because the planet has less gravity.
Part of the mission in orbit consisted of experiments that will determine the program for landing humans on Mars. Scott completed every field test designed to simulate what astronauts landing on Mars would experience.
Most difficult for Scott and Kornienko were muscle dexterity, postural control and stability during the Functional Task Test. They had to be able to climb ladders, jump down, open hatches, use tools, recover from a fall, avoid obstacles, among other things. But there wasn't a substantial difference between their results and those of astronauts who have been in space for only six months.
Researchers also looked at how the men's fine motor skills held up after a year in zero gravity by having the astronauts point, drag, trace shapes and pinch rotate items on a tablet computer screen, similar to how astronauts may need to rely on devices after landing on Mars. The early results suggest that accuracy and reaction time decreased.
The astronauts differed in their recovery times after landing, but that could be due to pre-flight training or experience in general. For now, researchers believe that more training to help readjust to the weight of Earth's gravity would help future astronauts, as both men experienced mobility changes with a slower recovery time than six-month mission astronauts.
Scientists have long aimed to improve astronauts' sleep duration and quality. With improved scheduling, fewer work shift changes and the fact that space station construction is completed, the one-year astronauts got an average of seven hours of sleep each night. Astronauts with shorter missions on the station averaged about six hours of sleep.
All of the results are preliminary findings, and research continues. A summary and in-depth research articles are expected later this year.
"The preliminary results from the year-long ISS expedition and the associated Twin Study ... have not identified any show-stoppers for longer human spaceflight missions," NASA Human Research Program chief scientist John Charles said in a statement.
"Some of the results, such as the unexpected increase in telomere length in Scott Kelly, require additional analysis and correlation with the results of other investigators, and may prove to be an artifact or transient change. Overall, the preliminary results are reassuring that a year in space is not significantly more stressful than 6 months in space. However, more definitive insights must await the further analysis of the samples and supporting data, the correlation of findings among all the investigators, and future long-duration ISS missions with these and other genetic investigations -- although, almost certainly no more twins, unfortunately."