Scott Kelly orbited the Earth for 340 days in the International Space Station and collected data about himself. He was drawing his own blood. He kept the urine. He played games to test his memory and reaction speed.
Nearly 400 kilometers down, Kelly's twin brother Mark, who was also an astronaut before retiring in 2011, was conducting identical tests. Now, comparing the collected data has provided a unique opportunity to finally find out exactly what happens to the human body in space - and even at the molecular level.
Three years after Scott Kelly (55) returned to Earth, NASA researchers announced on April 11 that his body had undergone a significant number of changes. Some of his cells had mutated DNA. His immune system was producing a large number of new signals. New types of bacteria have been found in the microbiome of astronauts.
Some of these biological changes seemed harmless, and most of them disappeared upon return. Others - among them genetic mutations and the weakening of cognitive functions - have not subsided, and scientists have grown concerned.
Some believe the risks can be kept under control, while others question whether it will ever be safe for astronauts to take long journeys to Mars, or beyond. There is much about life in space that scientists still cannot explain. NASA, however, hopes that with research like this with the Kelly twins, they will be able to find answers to some outstanding questions.
In 2012, NASA selected Scott Kelly to join Russian cosmonaut Mikhail Kornienko aboard the International Space Station for a year-long examination of the challenges of space travel, twice as long as any similar journey by astronauts before.
Kelly says his idea was to compare himself to his twin brother.
"We have these two guys who are genetically identical," Scott recalls saying when he made his proposal. "That would be an interesting experiment."
By comparing the two twins, NASA is trying to better understand the changes Scott Kelly experienced during the mission.
"The fact that they are twins really narrows down the alternatives," said Dr. Susan Bailey, a biologist at the University of Colorado and one of the authors of the new study. "We could say that the resulting changes were caused by living in space."
Scott's body changed in a surprising way. Susan Bailey studied special parts of his DNA called telomeres, which are located at the ends of chromosomes and protect them from degradation. When people age, telomeres shorten. Stress - like, say, radiation - can accelerate aging so that telomeres shorten even faster.
Scott's average telomere length in space increased, as if his cells had rejuvenated. The universe may have awakened the stem cells in his body, Bailey believes. It is possible that his body produced a new supply of young cells with longer telomeres.
Going into space seems to have triggered a genetic change in Scott as well. Thousands of dormant genes were suddenly activated - while the same genes in Mark Kelly's body on Earth continued to be dormant. The longer Scott Kelly stayed in space, the higher the number of active genes became.
Some of the awakened genes stimulate the creation of proteins that contribute to the repair of damaged DNA. That would actually make sense, since the radiation level in the International Space Station is higher than on Earth. Christopher Mason, a geneticist at the New York School of Medicine, estimated that Scott Kelly was exposed to radiation 48 times stronger than the average strength of radiation on Earth over a period of one year. His cells had to repair the effects of the radiation faster.
Other genes that were activated in space play important roles in the immune system, but it's still not entirely clear why. Recent research, however, also shows that latent viruses can be reawakened in astronauts. Or the immune system just gets confused in space.
Kelly's return to Earth on March 1, 2016 turned out to be one of the most important moments of his mission, at least in biological terms. His body showed signs of intense stress, and his immune system went into full alert.
Michael Snyder, a geneticist from Stanford University in California and one of the authors of the research, warned that this kind of reaction is not entirely typical and expected.
"Maybe he got a viral infection," Snyder speculated. "That's why it's necessary for more people to go through this."
Kelly's body was mostly back to normal upon returning to Earth. Some types of bacteria that developed in his stomach while he was in space are now back in traces. The strange lengthening of his telomeres disappeared after less than 48 hours. What's more, the scientists found that the telomeres in many cells had actually become shorter than they had been before Scott went into space.
"People have asked me, 'Well, is going into space a fountain of youth?'" says Dr. Susan Bailey. “I don't think so. And if you did, you would have to stay there forever”.
Some aspects of Scott's biological condition did not normalize, however. Six months after he returned from space, 8,7 percent of his genes are still functioning completely differently. Geneticists describe this change as moderate.
Scott also did not do well on cognitive tests upon his return.
"He was slower and made more mistakes on almost every test," says Dr. Matthias Basner, a cognitive scientist at the University of Pennsylvania.
It is possible that a biological change is to blame for this, but Dr. Basner points out that many demanding things awaited Kelly upon his return to Earth, such as numerous interviews and speeches, and it is possible that, on an unconscious level, he no longer pushes himself.
"Why would anyone want to go to Mars or spend time in space?" says Dr. Eric Tull of the California Research Institute. “Because it's very scary”.
Jerry Shay, a cell biologist at the University of Texas, hopes to be able to protect the astronauts with certain countermeasures. He and his colleagues are already testing drugs that will help cells repair DNA damage from radiation.
"I think all these problems are solvable," says Dr. Shay.
The New York Times
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