At a press conference not long thereafter, it was Scott who hinted that that this mission might provide the chance to compare the impact of space living on his body with his Earth-dwelling identical twin brother, Mark Kelly, who had also been an astronaut and former Navy test pilot.
Bailey's project was one of 10 investigations supported by 84 researchers across 12 universities, all coordinated by NASA's Human Research Program.
Christopher Mason, associate professor at Weill Cornell Medicine and principal investigator of gene expression, said many genes began to change right when Kelly got to space, but that was expected because the human body is "always changing".
Among his changes, about 7 percent persisted after six months on Earth, according to NASA.
NASA astronaut Scott Kelly spent a year on the International Space Station. However, because the ISS is not traveling anywhere near the speed of light relative to us, time dilation - or the slowing of time due to motion - is very minimal.
As part of the study, more than 300 urine, fecal and blood samples were collected from the twins.
At the same time, scientists monitored Mark - a veteran himself of four Space Shuttle missions - on Earth as what they called a "genetically matched ground control".
This study - described by study coauthor and genomics expert Andrew Feinberg as the "dawn of human genetics in space" - is unquestionably valuable.
"Diversity remained constant for Scott during his time in space, and this is, from my perspective, a positive finding suggesting substantial resilience and robustness in the gastrointestinal microbiota", he said.
Mark did not have any such thickening.
WhatsApp will let you ignore people without blocking or exiting groups
This feature is important because until now archived chats would be unarchived every time you received a message on that chat. Moreover, the archived chats automatically become un-archived when the users will be receive a new message from the sender.
Teams of researchers from around the nation analyzed the data and report their findings on the biochemical, cognitive, ocular, genetic, physiological, immunological and other changes in the Science paper.
Weight loss, altered eye shape and elongated DNA structures are just a few of the side effects of spending a year in space.
The gut microbiota helps digest food, fights infections and plays an important role in keeping the immune system healthy. "Many astronauts develop SANS-related vision impairment that may be the result of multiple hits on the vascular system involving microgravity-related fluid shifts, environmental changes, and possibly a genetic pre-disposition". "And these are the same genes that we see changing in cancer patients", said Piening.
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.
"There may be other things coming down the pipe when we consider three-year missions to Mars", said Michael Bungo, a professor of cardiovascular medicine at the University of Texas Health Science Center. Telomeres typically shorten as a person ages. But much to our surprise, Scott's telomeres were significantly longer at every time point and in every sample tested during spaceflight. Among them: gene expression, telomere length, cognitive performance, microbiome composition, retina thickness, and metabolite production.
Bailey cautioned that the finding "can't really be viewed as the fountain of youth and that people might expect to live longer because they're in space". But that's no fountain of youth, the study found, because the telomeres shortened dramatically when he returned to Earth. While that doesn't directly equate to an increase in heart disease risk, said Mishra, it's a potential factor to watch for in future studies of the effects of long-term space exposure on humans.
Bailey said her team has not come up with an explanation for the telomere lengthening but was looking at whether the higher radiation exposure in space, inflammation or stress may be responsible.
"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.
On one occasion, supplies for the study being shipped to the ISS were lost when a SpaceX transport rocket blew up.