The year in space has put the immune system of the American astronaut on the alert with respect to his land twin.

Nearly a year in space has put the immune system of astronaut Scott Kelly in a state of alert and has changed the activity of some of his genes compared to his identical twin linked to the Earth, announced Friday researchers.

Scientists do not know if the changes were positive or negative, but the results of a unique study on NASA twins raise new questions for doctors, with the space agency wishing to send people to Mars .

The double genetic tests have given scientists a unique opportunity to follow details of human biology, such as how an astronaut's genes activate and deactivate themselves differently in the space that at their home.

A confusing change announced Friday at a scientific conference: Kelly's immune system was hyperactivated.

"It's as if the body was reacting to this extraterrestrial environment a bit like a mysterious organism was in you," said Christopher Mason, geneticist of Weill Cornell Medicine, of New York, who helped lead l & # 39; study. He said that doctors are now looking for that in other astronauts.

Since the beginning of space exploration, NASA has been studying the harmful consequences on astronauts' bodies, such as bone loss requiring physical exercise. In general, they are in space about six months at a time. Kelly, who lived on the International Space Station, spent 340 days in space and set an American record.

"I have never felt completely normal in the space," Kelly said today, in an email to the Associated Press.

But this study was a unique dive into the molecular level, with former astronaut Mark Kelly, Scott's twin, on the ground for comparison purposes. The full results have not yet been released, but the researchers presented their findings Friday at a meeting of the American Association for the Advancement of Science.

A number of genes related to the immune system have become hyperactive, Mason said. This is not a change in DNA, but in what is called "gene expression", how genes turn off and increase or decrease their protein production.

Mason also spotted a tip in the bloodstream of another marker that stimulates the immune system. However, at the same time, Kelly's blood had less of another type of cell that constituted an early defense against viruses.

It is not surprising that gene activity changes in space – it changes in response to all kinds of stress.

"You can see the body adapt to changing its environment," Mason said.

The good news is that most things returned to normal shortly after Kelly's return to Earth in March 2016. These immune-related genes, however, "seemed to have that memory or need to be almost in a state of alert ", six months later still, Mason told me.

"Overall, it's encouraging," said Craig Kundrot, head of NASA's Space and Science Life Research. "There are no major new warning signs. We are seeing changes that we did not necessarily anticipate, but we do not know if these changes are significant.

NASA already knew that it was possible to extend the stay on Earth of four Russians living in space for more than a year, said Kundrot, adding: "We are also aiming more that all that is possible. We want our astronauts to do more than survive.

In the end, the twin study provides NASA with a catalog of points to watch for future missions to determine if other astronauts are responding in the same way. Future mission astronauts will be able to do some of these tests in space instead of freezing samples for scientists returning home, Mason said.

Immune problems seem familiar to Dr. Jerry Linenger, an American astronaut who spent more than four months on the Russian space station Mir. He said that he was never sick in orbit, but once back on Earth, he was probably sicker than in my life.

The astronauts launch into orbit with their own germs and are exposed to the germs of their teammates. After a week without anything new in the "very barren environment" of a space station, your immune system is really not challenged, "said Linenger.

A human mission on Mars, which NASA hopes to launch in the 2030s, will last 30 months, including time on the surface, Kundrot said.

Radiation is a major concern. The mission would expose astronauts to levels of cosmic galactic radiation that exceed NASA's safety standards. It's just a little over, he says.

On the Earth and even on the space station, the Earth's magnetic field protects astronauts from a lot of radiation. There would be no such armor on the way to Mars, but tunnels or earth-covered habitats could help a bit on Mars, Kundrot said.

Kelly, who will turn 55 next week, said that he would go on Mars. He said that such a long trip would not be worse than what I have experienced. Maybe better. I think the big physical challenge, radiation aside, will be a mission where you will stay in space for years.