A large, interdisciplinary research team tracked the health and biology of both men, in a groundbreaking attempt to observe the effects of spaceflight on the human body.
There's a lot in space that can affect your health. Radiation, space station air quality, zero gravity and the stress of being trapped in a closed environment far from home can all take a toll.
What researchers discovered is pretty reassuring for near-Earth space travel -- while Scott's body did undergo some changes compared with Mark, things went back pretty much to normal once he returned home.
"It's reassuring to know that when you come back things will largely be back to the same," said researcher Michael Snyder, chair of genetics at Stanford University School of Medicine. "That's the No. 1 message."
For example, Scott experienced a large-scale shift in the way his genes expressed themselves while he was in space, but it didn't seem to affect his health and largely went back to normal upon his return, researchers said.
"We saw that the vast majority, over 90 percent, of all these changes all returned back to baseline coming back to Earth," said researcher Chris Mason, an associate professor of genetics with Weill Cornell Medical College in New York City.
Changes to cellular aging?
The most lasting change occurred in Scott Kelly's telomeres, the protective caps on the end of chromosomes, said researcher Susan Bailey, a professor with the Colorado State University Department of Environmental and Radiological Health Services.
Think of telomeres as similar to the plastic tips on the end of shoelaces -- they keep chromosomes from unraveling and causing damage to healthy cells. Shorter telomeres put a person at higher risk for accelerated aging, and for age-related diseases like heart problems and cancer, Bailey said.
Space travel had an elastic effect upon Scott Kelly's telomeres that his brother Mark didn't experience, despite their identical genetics.
"Scott's telomeres were longer during spaceflight than they were either before or after," Bailey said. "We also saw a very rapid decrease in telomere length upon his return to Earth."
Scott Kelly continues to carry shorter telomeres after flight than he had before, Bailey added.
"From the perspective of aging and health risks, that could be where he might be at increased risk for cardiovascular disease, for example, or some types of cancer, too," Bailey said.
Unprecedented collaborative study
The idea for the NASA Twins Study came from Scott Kelly himself, during preparations for his 340-day mission in space with Russian cosmonaut Mikhail Kornienko, said co-lead investigator Dr. Andrew Feinberg, director of the Center for Epigenetics at Johns Hopkins University.
Kelly and Kornienko left Earth for the space station in March 2015 and returned in March 2016.
The Twins Study encompassed 10 separate research teams that thoroughly tracked the health and biology of both Scott in space and Mark on the ground.
Some other findings from the study:
* Scott's carotid artery wall became thicker during the mission, which is considered a risk factor for heart disease and stroke, said researcher Stuart Lee, lead scientist for KBRwyle, a contractor supporting the cardiovascular and vision laboratory at NASA Johnson Space Center.
* A flu vaccine administered in space worked exactly as it would on Earth, indicating that the immune system is not significantly compromised outside the planet.
* Changes in the makeup of Scott's stomach bacteria in space were no greater than stress-related changes scientists observe on Earth.
Scott experienced some changes to his vision and the shape of his eyeball, including a thicker retinal nerve and folds in the choroid layer that surrounds the eye, possibly due to the effects of zero gravity on fluids in the body.
About 40 percent of astronauts experience these sort of vision changes, said Lee. It's occurred in other male astronauts, but not females.
Researchers expect that "those are probably some permanent changes that won't resolve over time," Lee said.
'Deep space' effects still unknown
This study provides a first glimpse into the effects of space travel on humans, but cannot predict what might happen to people as they venture into deep space, the researchers cautioned.
"This is not the environment that the astronauts are going to be facing when they go to Mars," Feinberg said, noting that the International Space Station is close enough to Earth to shield it from deep space radiation. "We don't have much experience with people leaving that protective shell."
"We need to get outside of low Earth orbit and we need for the astronauts to spend longer periods of time to really evaluate some of these health effects," Bailey added. "The radiation exposure will certainly be a big concern as they get outside of the protection of the Earth."
The findings from the study were published Friday in the journal Science. In a video, Bailey outlines her research with the twins on the effects of space on the human body:
NASA has more on humans in space.
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