In a landmark study, NASA scientists conducted exhaustive tests on Scott Kelly during his grueling 342-day tour aboard the International Space Station in 2015–2016. And they simultaneously monitored his identical twin brother Mark (a retired astronaut who was on Earth at the time), hoping the comparison would reveal new information about the effects of long-term spaceflight on the human body.

The results of the NASA Twins Study, reported Thursday in Science, show Scott Kelly underwent a number of physiological changes—most of which returned to pre-flight status after he returned to Earth. The findings could help inform preparations for future long-term missions, such as travel to Mars or stints on a moon-orbiting space station.

The study was the first to integrate both behavioral analysis and physiology to investigate how space influences a person, and the first to collect two years of such comprehensive health data on a middle-aged Earthbound man, says co-author Brinda Rana, a molecular geneticist at University of California, San Diego.

“[This is] science at its best, in many ways, because it is leveraging something that is naturally occurring to try and understand causality,” says behavioral geneticist S. Alexandra Burt, who co-directs the Michigan State University Twin Registry and was not involved in the study. “I think this was a unique event and they capitalized on it beautifully.”

Kelly says he initially proposed the study while preparing for the ISS mission. “I said, ‘Hey, if anyone asks about any studies planned with my brother Mark and I, because we’re identical twins, is there any interest in that?’” he says. He was initially rebuffed, but he says NASA scientists eventually spoke to several universities “and they found out there was actually a lot of interest.” The Twins Study was a go.

For nearly two years both brothers provided urine, stool and blood samples, which researchers used for 10 separate investigations involving the molecular, physiological and cognitive effects of long-term spaceflight. Close coordination among the many investigators was essential. “NASA got 10 investigators from around the country—most of whom didn’t know each other—and put us in a room and got us to all cooperate and become a cohesive team to carry out this mission,” Rana says. The inherent difficulties of collecting such specimens from an astronaut aboard the ISS meant the investigators had to work with relatively scant material. “We had to come up with protocols where we could all do our studies effectively with one sample,” Rana says. The study also spurred researchers to innovate new methods for studying physiology and genetics in space. “It’s helped us drive technology to do these sorts of experiments not just for low-Earth orbit, but for future activities [such as] exploration missions,” says co-author Stuart Lee, a cardiovascular scientist at KBRwyle.

Kelly had to draw his own blood and collect his own urine and stool. “Certainly it’s more complicated to pee in a bag in space, because everything floats,” he notes. “But most importantly, it takes time. Astronauts sometimes find themselves almost subconsciously trying not to be as hydrated on urine collection days because of the time it takes to pee.” This was reflected in the results: researchers found Kelly was consistently mildly dehydrated, a common phenomenon among astronauts on long-term missions—and scientists do in fact attribute this to astronauts’ annoyance with the complicated onboard lavatory.

The study found Kelly was under near-constant physiological stress while in flight. His immune system was on high alert; he had changes in DNA expression; his carotid artery became distended. He also developed “spaceflight-associated neuro-ocular syndrome,” or SANS—a condition that involves changes to his eyes, and for which the cause is poorly understood. And his blood, no longer drawn to his legs by gravity, collected in his head. “In the beginning, it’s the worst,” Kelly says. “Your body adjusts to it over time, but in my experience, it never adjusts completely. I always felt pressure in my head.”

While in flight, Kelly’s telomeres—the protective sequences of repeating nucleotides that “cap” the ends of each chromosome like aglets on a shoelace—grew longer. “That certainly was a surprise, because going into the study we had proposed exactly the opposite,” says Susan Bailey, a biologist at Colorado State University who directed the study’s telomere investigation. Telomeres shorten as we age, and an enzyme called telomerase repairs and lengthens them. Astronauts are exposed to extreme stresses of microgravity, isolation and radiation that should contribute to telomere shortening, Bailey says. She suspects Kelly’s highly regimented diet and exercise program aboard the ISS may have contributed to the lengthening—or that the phenomenon may indicate his cell population was changing during spaceflight. “Going forward, I think we will try to incorporate these telomere endpoints into some of the standard measurements that they’re doing on the astronauts, so that we could actually address that question directly,” she says.

After Kelly returned to Earth, most of the physiological, gene expression and other changes returned to normal within about six months. But there was still a period of adjustment. “When you get back to Earth all the blood just wants to pool in your legs,” he says. “That lasted for me for weeks.” He also developed rashes and hives whenever his skin, which had grown unaccustomed to gravity, had any pressure on it. Even leaning on a table caused a reaction. But he did eventually readjust, a hopeful sign there may be little lasting danger to astronauts’ health after long periods in space. That said, it is important to note that traveling to Mars or other long-distance explorations presents an additional concern: radiation. Astronauts aboard the ISS are still somewhat protected from cosmic rays by the Earth’s magnetic shield; a Mars mission would expose them to galactic radiation at much higher levels, Lee says.

The study authors caution that given the extremely small sample size, it is “impossible to attribute causality.” They write that the study should be “hypothesis-generating and framework-defining,” with future research on astronauts complementing the work. “That’s all true,” Burt says. But she adds that having a genetically identical subject back on Earth allows for assuming causality much more strongly than the study otherwise could have. Kelly agrees: “Granted, this is an experiment with an n of one,” he says. “But they didn’t see anything that would prevent us from going to Mars. However, he adds, “if we’re going to go beyond Mars”—requiring astronauts to stay in microgravity for years on end—“we’re going to have to start thinking about artificial gravity.”