Space is doing some strange things to astronauts' bodies, a new study has found.

The Nasa twins study is looking at two twins – Scott and Mark Kelly – in an attempt to understand the affects of spaceflight on people's health and physiology. By having one remain on Earth while another lived on the International Space Station for more than a year, researchers are able to compare the two in an attempt to understand what difference to the trip into space made.

Now early results show a host of unusual changes happened to Scott when he headed into space. That included the discovery that in some ways, a person's health is not greatly affected at all by a trip into space.

Perhaps most unusually, researchers looked at how astronauts aged when they were on board the International Space Station. They did so in part by looking at the pair's telomeres, or the protective caps that sit on the end of chromosomes and reflect the ageing process in the body, since they typically shorten as a person ages.

In fact, though Scott's telomeres lengthened out when he was in space, a discovery that shocked the researchers.

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"We were surprised, that was the first reaction," said Colorado State University professor Susan Bailey. "But that's what science is all about, right?"

But they then also found that they dramatically lengthened again when getting back down. While most of them settled back down again over time and have gone back to normal, he now has more short telomeres than when he set off, suggesting that spaceflight could be associated with age-related conditions.

Otherwise, the study found that the effects of the year-long flight were largely similar to time spent in any other stressful environment. There were changes in Scott's gene expression levels and his gut flora, for instance, but after six months back on Earth they had settled back to normal.

"Given that the majority of the biological and human health variables remained stable, or returned to baseline, after a 340-day space mission, these data suggest that human health can be mostly sustained over this duration of spaceflight," the authors wrote.

Studying the identical twins, who were 50 years old at the start of the study, gave scientists a unique opportunity to compare individuals with the same genetic material subjected to different environments.

Exposure to radiation, lower gravity, less exercise, a restricted diet and disrupted sleep are among the possible risks of spending time in space.

Mark is a fellow astronaut and has also been to space before, but not for almost four years prior to Scott's mission.

"The twins study is certainly the most comprehensive view that we've ever had of the response of the human body to space flight," Professor Susan Bailey, from Colorado State University, said.

Blood samples, physiological data and cognitive measurements were taken from the brothers at regular intervals over 25 months.

While Scott was in space, his samples were sent back to Earth with shipments that had delivered supplies by rocket to the ISS.

Changes to the shape of Scott's eyeball, including a thicker retinal nerve, were among those detected, as well as a decline in some of his cognitive abilities.

The scientists also found that the astronaut's telomeres - the protective caps on the ends of chromosomes - got longer while he was in space.

Telomeres typically shorten as someone becomes older and are associated with cardiovascular diseases and some cancers.

However, many rapidly decreased in length after the flight, and he now has more short telomeres than long ones.

There were also changes in the expression of some of Scott's genes, particularly those related to the immune system.

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However, more than 90 per cent of these had returned to normal six months after the flight.

Persistent changes were observed in a few other areas, including some cognitive function.

"There are dramatic changes, there are thousands of genes and molecular changes, that occur as someone goes in space," Dr Mike Snyder, from Stanford University School of Medicine, said.

"I think it's reassuring to know that when you come back things will largely be back to the same."

Scientists believe the findings could help predict possible health complications during long-term missions.

"If we know what to expect, we can anticipate health problems astronauts may encounter and ensure that medicines and other remedies are at hand during a mission," Dr Andy Feinberg, at Johns Hopkins University, said.