NASA's landmark study followed the lives of astronaut Scott Kelly and his twin brother Mark, a retired astronaut, in an attempt to understand how long-term space travel may impact the human body. For more than a year, Scott lived aboard the International Space Station (ISS) while his brother lived on Earth for baseline observations.

Now, a whopping 10 studies compiled into one comprehensive paper published in the journal Science finds that the environmental factors of living in space affect the human body in a number of ways, including gene expression, bone density, immune system responses, and telomere dynamics.

"There were ten teams of researchers but only one article," said microbiome study author Martha Vitaterna in a statement. "This multi-system, integrated analysis over time is one thing that makes the Twins Study unique and powerful. We can see which things change together in the human body."

Altogether, those changes are an attempt by NASA to better understand how astronauts and space tourists alike might withstand long-term spaceflight, particularly as the agency plans to send people to Mars by 2035. 

Here's what we know.

Extended Spaceflight Affects The Human Gut Microbiome

During his 340-day space tenure, Scott experienced a shift in the ratio of two major types of bacteria in his digestive tract.

The Northwestern-led team looked at how Scott’s gut bacteria composition changed over time by examining fecal samples collected before, during, and after his time aboard the ISS. They found the ratio between Firmicutes (good bacteria) and Bacteroidetes (bad) shifted dramatically, with the former increasing while the latter decreased. Though it was among the greatest compositional changes his body underwent in space, these levels returned to normal when he came back to Earth.

The researchers note that a number of things could have caused these alterations, including microgravity (the most likely culprit), increased radiation, shifts in sleeping schedules, a lack of air circulation, as well as stress and an altered diet made up of mostly freeze-dried pre-packaged food. 

Extended Time In Space May Mean A Person Is At Risk For Accelerated Aging Or Related Diseases

Colorado State University researchers noted that Scott's telomeres, protective "caps" on the end of chromosomes, were longer in white blood cells while he was in space, even though they typically shorten as a person ages.

By analyzing blood samples shipped via a Soyuz rocket during the mission, as well as samples collected before and afterward, the team discovered that he has more short telomeres than before his 340-day mission, putting him at a higher risk for accelerated aging and related issues such as cardiovascular disease and some cancers. 

No Lingering Major Epigenetic Differences Were Observed

No major differences between the two men's epigenomes, the body's record of chemical changes to DNA, were recorded by scientists at Johns Hopkins Medicine.

Collected blood samples, physiological data, as well as cognitive measurements were again measured from the twins over the 27 months before, during, and after the mission. Results indicate that no major epigenetic changes – chemical "tweaks" to DNA that may influence gene activity but not change the code itself – occurred. In fact, scientists found that just as many changes occurred in both men, with a less than 5 percent difference overall in methylation during the mission. The largest difference occurred nine months into the study when 79 percent of Scott's DNA was methylated compared with 83 percent of Mark's. 

The results are "encouraging", but of course are limited by the fact that there are only two participants in the study. 

Long-Term Spaceflight Causes More Changes To The Immune And DNA Repair Systems Than Shorter Trips

Researchers at Weill Cornell Medicine searched for mutations or damages in the men's DNA in order to determine which genes were turned on or off based again on changes to epigenetics. Additionally, they looked at changes in proteins produced by the men's cells to see how the changes might affect their health. They found that mitochondrial genes, which help the body produce energy and boost immunity, changed in terms of activity, indicating cellular stress. Gene expression also changed dramatically, with six times more changes occurring in the last six months of the mission than the first. Many of these changes stayed the same, but some like cognitive deficits, vision impairment, and T-cell activation remained. 

Microgravity And Environmental Changes May Impair vision

Scott exhibited a thickening of the retina known as space-associated neuro-ocular syndrome (SANS) possibly due to “multiple hits on the vascular system involving microgravity-related fluid shifts, environmental changes, and possibly a genetic predisposition," said senior author Brinda Rana.

Researchers at the University of California San Diego analyzed biofluids for information on metabolomics, proteomics, and mitochondrial function and found that increases in collagen proteins observed during space flight correlated with physiological measures that indicated "vascular remodeling".