What NASA’s twin study actually taught us about living in space

Results show the human body changes, but not dramatically.
Twins Scott Kelly and Mark Kelly will have their vision compared NASA/Robert Markowitz

Twin studies are the closest scientists can get to putting a person in two places at once. But they are also hard to come by: Identical twins are relatively rare (just three to four per thousand births worldwide). Even rarer are identical twin pairs that are astronauts. In fact, right now, just one set exists: Scott and Mark Kelly.

Over the past few years, the brothers have participated in what’s now called the NASA Twin Study: A first-of-its kind longitudinal study in which one brother (Scott) spent a year onboard the International Space Station (ISS) and the other (Mark, who had already retired from spaceflight) spent those same 12 months on Earth. That year, and in the ones that followed, the pair provided blood, urine, and stool samples—in addition to undergoing a slew of physiological and medical tests—to help researchers understand how the human body responds to life in space. The most recent results were published this week in the journal Science.

A major roadblock to understanding how space affects the human body is that it’s incredibly difficult to blame any changes that occur on spaceflight alone. For example, a slight mutation in an astronaut’s DNA could have been from time spent in zero gravity or excess radiation, or it could be from any number of environmental conditions that would also have affected them on Earth. But if that specific change is seen in one twin who spent a year in space (340 days, to be exact) and no comparable change happened in the twin who spent the same year on Earth, researchers can say with a bit more confidence that space had something to do with it. This is still far from a perfect way of pinpointing space-related bodily changes, because a study with just two people in it is awfully small. But it’s better than comparing an astronaut to the average Earthling.

The ultimate question is how the human body will hold up during long term space travel. NASA has tentative plans for missions to Mars and trips back to the moon, which would mean more time in space than quick jaunts to the ISS have prepared us for. Knowing if our bodies can stand the strain is crucial.

So what does a year in space actually do to the body? Overall, the results of the twin study suggest that for a year’s time, “human health can be mostly sustained over this duration of spaceflight.” Many changes were temporary and slowly stalled upon Kelly’s return to Earth, while others have stuck around. Understanding all of these changes will take time, but here’s what we know for now.

Length of telomeres:

Telomeres are molecules that sit at the end of DNA segments and protect them from damage and degradation. Aging, as well as other factors like stress and environmental changes, can speed this process up. Some researchers speculated that the stressor of space would cause Kelly’s telomeres to deteriorate. However, the opposite turned out to be true: They lengthened. Researchers saw an increase in the activity of the proteins that regulate telomere length—higher than both before he left for space and upon his return.

Why that is is not entirely clear. Researchers know that following a healthy diet and exercise regimen—which Kelly did while on board the ISS—could have this effect on the telomeres. Astronauts stick to strict nutritional plans and workout routines to fight back against the muscle and bone atrophy of space, so some of them might actually have healthier lifestyles while afloat than they do on the ground. It’s also possible that the drastic shift in environmental conditions caused his body to produce new cells, with longer telomeres, as a protective mechanism.

Gene activity changed:

The activity of our genes—which results in permanent alterations to our DNA—change all the time. When this happens some proteins become more active and some less. It’s part of the aging process, and universal to humans. Certain gene changes can lead to disease while others can lead to an increase in fitness or cardiovascular health. But gene activity can also change when people are exposed to radiation, which space travel has a ton of due to its lack of protection from Earth. Scott Kelly saw an increase in activity in many genes that remained inactive in his brother Mark.

While scientists don’t know for sure, they surmise that this shift in activity is at least partially related to the amount of radiation exposure Scott Kelly received compared to his brother. An increase in radiation exposure may cause cells to attempt to repair themselves, thereby increasing certain gene activity.

Other genetic changes, like DNA mutations, were also present in Scott Kelly that didn’t exist in his brother. While those changes aren’t immediately concerning, their activity over time could increase the chances of cancer. This could be far more concerning when people spend longer than a year in space, as these mutations accumulate.

Altered body bacteria:

The microbes that live in and on our bodies—what scientists call the microbiome—can have a significant effect on our health. They affect digestion, influence our metabolism, and can also play a role in our immune, bone, muscle, and brain health. To understand how our resident microbes survive in space, researchers took stool samples from Kelly before, during, and after his days in space and compared them to those of his twin.

They found that while gut bacteria in particular did change drastically while he was in space, the microbiome quickly shifted back to more normal levels upon his return to Earth. He didn’t experience any major loss of any bacterial diversity—which scientists think is a crucial part of a healthy microbiome—during the process. And the fact that things shifted back to normal quickly once he came back home suggests that this gut upheaval wouldn’t have serious longterm effects. (For context, previous studies have shown that gut microbes can shift significantly after just a few weeks of eating a different diet or living in a different place.)

What now?

With all of this new information, the researchers write, they can begin to ask more specific, directed questions about how the body adapts to space—and begin to better answer whether or not it’s safe or wise for humans to travel to Mars. Researchers can also work on ways to combat these changes, either through novel drugs or other interventions, with the ultimate goal of making all spaceflight—no matter how long its duration—safer and less mysterious.