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What NASA's Extraterrestrial Twin Experiment Showed Us

#21 in our top science stories of 2019.

By Jake Parks
Dec 25, 2019 10:00 PMJan 2, 2020 2:31 PM
Astronaut Scott Kelly spent a year aboard the International Space Station while his identical twin stayed Earth-side, allowing researchers to study how spaceflight affects the body. (Credit: NASA/Robert Markowitz)


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Every grade-school scientist knows a good experiment needs a control — a test subject you leave alone to have a baseline for comparison. So when NASA researchers set out to learn exactly how weightlessness and other space hazards like radiation might change the human body, they needed someone to stay behind on Earth as a counterpart. The ideal control would be someone very similar to the space traveler, so NASA chose an actual clone — or, as they’re more commonly known, an identical twin.

Thus was born NASA’s Twins Study. From 2015 to 2016, astronaut Scott Kelly spent a year aboard the International Space Station. Meanwhile, retired astronaut Mark Kelly — Scott’s twin brother — stayed on Earth. The results, published in April in Science, show spaceflight indeed triggers changes in the human body, such as damaging DNA, thickening artery walls, modifying the microbiome and altering gene expression. But the vast majority of these changes disappear within a few months of returning to Earth.

Through the Twins Study, researchers now have a better understanding of the hazards of spaceflight and can work on how to deal with them. The results may eventually help us venture to the moon, Mars and beyond. We spoke with Scott Kelly about the findings, the possibilities for long-term space voyages and exactly what space smells like.

Q: What do you think will be the most challenging hurdle for future astronauts embarking on a long-duration trip to Mars? 

A: I think the biggest risk is radiation. We need to know how to protect the crew from it. We need to know the implications of radiation on our physiology. There are other challenges, too. We had issues with our vision [in space]. We had issues with deconditioning after being in space for a long time. I think for longer-term spaceflight — like if we go to the outer planets of the solar system, where you'll have people in space for years — then artificial gravity is a requirement.

Kelly (right) faces off with his twin brother, retired astronaut Mark Kelly. (Credit: NASA/Robert Markowitz)

Q: So we should be optimistic about long space trips in the future? 

A: I think what NASA’s advertised as the big [takeaway of the Twins Study so far] is that we got a lot of data. There are areas we need to investigate further, like a lot of genetic stuff. This is the first time we have ever done any kind of genetic-based research in space on humans. There are things with gene expression that I think they want to dig into further. But overall, the findings were: “Hey, there are no show-stoppers in going to Mars.”

Q: What about another aspect of being cooped up in one place for so long — the smell?

A: You know, every place has its smells. Generally, I describe the space station’s smell as a cross between an antiseptic smell, garbage and BO. Not completely objectionable, but it often depends on where you are. If you’re next to the wet trash that’s been sitting on the station for a few months, it smells more like garbage. If you’re by the cans that we put the solid waste in, it might smell a little bit like that. If you’re by an area that was just at vacuum [exposed to space] — like after a spacewalk — it smells kind of like burnt metal to me, like maybe a sparkler on the Fourth of July. But it does have a unique smell.

One time I was getting a tour of the Harris County jail — not because I did anything wrong, just a tour — and I walk into this room with a bunch of prisoners in there. I had this déjà vu. I was like, [sniffs], “Almost smells like the space station.”

Q: The Twins Study looked at the physiological impact that long-term spaceflight has on the human body. But how did your year in space affect your mental well-being? 

A: I would say the most challenging thing is you’re isolated. You’re in the same place every day. You have the same people. Even though they’re good people that you enjoy being with, there are a lot of other people on Earth that are important to you, so you miss them. You miss going outside. You miss the weather, the sun, the rain, the wind. When you go to sleep, you’re at work. When you wake up, you’re at work. Even though you like being there, you also like being home.

One of the cosmonauts, Gennady Padalka, says, “You know, as astronauts or cosmonauts, when we’re on Earth, we dream of space. But when we’re in space, we dream of Earth.”

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