After almost a year in orbit above the Earth on the International Space Station, astronaut Scott Kelly experienced extensive physiological changes, according to a study published today (April 11) in Science.
"The unique thing is that because they're twins, essentially they have the same genetic code", said Dr Andy Feinberg of Johns Hopkins University.
Astronaut Scott Kelly takes a cognition test while on the International Space Station.
While hundreds of humans have flown in space before, there is little data on how space flight longer than a few months impacts health and the body.
A NASA-coordinated study on two astronaut twin brothers showed that long-duration spaceflight may change an astronaut's gene expression and pose other health risks, but they are likely within the range for humans under stress. Some results had been reported in February.
Another genetic change had to do with the length of Scott's telomeres - that is, the molecular end caps on his chromosomes.
Having shorter telomeres puts a person at higher risk for accelerated aging, said Bailey. 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.
The study is exceptionally detailed ("They measured as many things as they possibly could", said Richard Gronostajski, a geneticist at the State University of NY at Buffalo), but when it's all distilled down, the message about spending a year in space - exposed to microgravity and mildly higher levels of radiation - is relatively clear.
Scott Kelly continues to carry shorter telomeres after flight than he had before, Bailey added.
"A lot of the genes that got activated were part of his DNA damage response, or his radiation response".
Scientists noted changes in the expression of Scott Kelly's genes while in space, with most - but not all - returning to normal after six months back on Earth. Along with his previous missions, he also holds the USA record for most cumulative time spent in space.
The flight also caused changes to the structure of Kelly's eye and thickening of his retina, which is experienced by about 40% of all astronauts. Other teams searched Scott's biological samples for biomarkers of atherosclerosis and upward fluid shifts in the body due to microgravity, which can affect vision and cause headaches. Scott's exposure to radiation in space, for example, led to minor mutations in his chromosomes.
NASA discovered that the telomeres of the Kelly brother who was in space got longer.
Scott also registered a slight loss in cognitive abilities when he returned to Earth, although it's not clear whether that's related to long-term spaceflight.
The "twins study" used Mark Kelly - a former astronaut who is now running as a Democrat for Senate in Arizona - as the control for a study of the effects of space travel on his brother, who spent nearly a year on the International Space Station in 2015.
NASA has a rigorous training process to prepare astronauts for their missions, including a thoroughly planned lifestyle and work regime while in space, and an excellent rehabilitation and reconditioning program when they return to Earth.
He said it probably took him six months once back on Earth before he felt 100% again, but acknowledged his wife said it seemed more like eight months.
Bailey's team evaluated Scott and Mark's telomere lengths before the flight and found that they were very similar.
Bailey said the uniqueness of using twins was because the scientists could "attribute any kind of differences that we saw in spaceflight to spaceflight".
As for trips to Mars, Mark Kelly said: "I hope it's sooner rather than later, and hopefully, our participation in this study will help us get closer to making a mission like that a success". The study will help inform future biomedical space research. 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.