Halfway home: Kornienko (left) and Kelly (center) have six months to go before they come home. Gennady Padalka (right) recently returned.
NASA
September 15, 2015 2:25 PM EDT

Scott Kelly is surely the only astronaut in history who couldn’t go to space without getting a tattoo first—two of them, in fact. You’d be hard-pressed to find Kelly’s tats, and that’s just how he wants it. There’s one dot on his neck and one on his arm—and if they don’t do much for body aesthetics, they’ve very valuable to science.

Kelly and his cosmonaut crewmate Mikhail Kornienko have just passed the halfway point in their planned year aboard the International Space Station (ISS), a marathon mission designed to test the human body’s ability to endure long-term space travel. One of the tests Kelly will be called upon to perform periodically is a sonogram of the carotid and brachial arteries to look for signs of atherosclerosis, the build-up of fatty plaques on arterial walls.

The sonogram probes have to be placed just so and it can take a long time for medics on the ground and astronauts in space to get the positioning right. So to save time, Kelly and a NASA sonographer visited a tattoo parlor on Earth, where the dots of ink were permanently applied at the right spots.

“He jokes those are the only tattoos he’ll ever get,” says Stuart Lee, lead research scientist at the Johnson Space Center cardiovascular laboratory. “I tried to get him to get a larger tattoo that said ‘Place probe here,’ but he didn’t go for that idea.”

MORE: TIME is producing a series of documentary films about the record-breaking mission to space. Watch them here.

That might be the only request Kelly refused in the run-up to the mission or the six months he’s spent aloft so far. His time in space will not be spent solely as a cosmic guinea pig. He is still slated for two space walks to help reconfigure the station for the arrival of commercial vehicles. And he and Kornienko, like all of the other crew members, have a daily workload of scientific and maintenance tasks.

But medicine is the biggest part of this mission, with Scott’s identical twin brother, retired astronaut Mark Kelly, serving as a one-man control group, undergoing many of the same medical tests on the ground. The studies are intended to look at how a whole range of body functions—vision, cardiovascular performance, bone strength, the immune system, the mind, the microbiome—are harmed by extended periods in zero-g, and how those effects can be reversed or at least slowed.

As challenging as the science itself is, it’s equally hard to figure out how to conduct your experiments at all when two of your subjects are 250 miles (400 km) overhead, spinning around the Earth at more than 17,000 mph (27,000 k/h). Kelly’s tattoos were one simple workaround, but others are more complex. Take blood sampling.

ISS crew members are trained at extracting one another’s blood, but the proper lab facilities to analyze the samples are on the ground. The first step in getting them where they need to go is first to spin the blood in a centrifuge while it’s still in space—which separates out of its various components. The samples are then frozen until they can be packed aboard Soyuz spacecraft that periodically bring crew members home.

The moment a Soyuz thumps down in Kazakhstan, frozen scientific cargo is transferred to a Lear jet and flown to Houston. The transfer time from space to Texas: 24 hours.

“It’s unbelievable because it’s faster than if you were on Earth and sending it to a clinical lab,” says Andy Feinberg, director of the Center for Epigenetics at Johns Hopkins University, and one of the bio-med researchers working on the year in space mission. “This pushes the space mission so it fits really well with how cutting edge research labs work.”

Studies of the microbiome—the ecosystem of bacteria, viruses, yeast and spores that populate the human gut—also rely on this kind of rapid shipment from space to lab, though in this case researchers rely on stool samples which, while decidedly less glamorous, at least require less technical expertise to produce and store. The microbiomes of both Kelly brothers will be studied this way over the year, with Scott eating a prescribed astronaut diet and Mark eating whatever he wants.

“Mark,” says Scott Smith of the Nutritional Biochemistry Lab the Johnson Space Center, “is free-range.”

Studying the immune system takes its own kind of imagination. During long-duration space missions, the body’s natural defenses can break down, partly due to oxidative and other metabolic stressors, and partly because in the controlled environment of a spacecraft, crew members are not exposed to all of the ambient organisms they would be on Earth. This may lead the immune system to let down its guard.

To compare how the Kelly brothers’ immune systems function in two different environments, both brothers received flu vaccines before Scott went to space, and their response was measured via blood samples. They will be vaccinated again while Scott is in orbit and once more a year after he returns.

“We take blood samples seven days after the vaccine because that’s the peak time for cells recruited by the immune system to do their work,” says Dr. Emmanuel Mignot of Stanford University, a mission scientist for the year in space project. “The risk exists that on a really long space mission the immune system could unlearn immunities so you could come back to Earth newly sensitive to pathogens you can usually shrug off.”

Kelly’s and Kornienko’s mission will not remotely answer all of the biomedical riddles space raises. Some of those answers may only come when human beings really do ship off for what would be a two-and-a-half year mission to and from Mars. There is absolutely no guarantee there won’t be some nasty surprises waiting—some “knee in the curve” as space doctors say. But every day spent in space at the close remove of low Earth orbit helps turn at least some of the unknowns into knowns. The more that happens, the safer future crews will be.

Write to Jeffrey Kluger at jeffrey.kluger@time.com.

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