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Everything We've Learned from NASA's Study of Identical Twin Astronauts

A year-long experiment involving Mark and Scott Kelly found that long duration space trips affect genes, eyeballs, and cognitive abilities.
​Mark and Scott Kelly. Image: NASA
Mark and Scott Kelly. Image: NASA

Identical twins Mark and Scott Kelly were selected by NASA to be astronauts in 1996 because the brothers had extensive experience as naval aviators. The twins were solid astronauts—they racked up missions on the Space Shuttle and then the International Space Station (ISS)—but the agency eventually recognized the unique scientific opportunity presented by two genetically identical astronauts.

This idea culminated in the NASA Twins Study, which was part of Scott Kelly’s One Year Mission onboard the ISS. From March 2015 to March 2016, the Kellys underwent the same medical procedures and tests in space and on Earth, with Earthbound Mark acting as the control to spacefaring Scott’s manipulated variable.


The goal was to study the genetic, physiological, and cognitive effects of space on the human body by studying blood, stool, skin samples from the twins, and observing their physical and mental states.

A comprehensive overview of the results from the Twins Study was published Thursday in the journal Science. Though preliminary research about the experiment has appeared before, the new paper outlines the major conclusions of the Twins Study over three years since Scott Kelly completed his year in space.

Dozens of authors contributed to the paper, which was led by Francine E. Garrett-Bakelman, an assistant professor of medicine, biochemistry, and molecular genetics at the University of Virginia.

“These results represent an integrated portrait of molecular, physiological, and behavioral adaptations and challenges for the human body during extended spaceflight and are important to individual astronauts and to many groups at NASA,” the team said in the paper.

“These data can be of immediate use by investigators and groups around the world planning future human spaceflight missions.”

Here are the most interesting takeaways from the new paper, which refers to Scott Kelly as the flight subject TW and Mark Kelly as the ground subject HR.

Space messes with our eyeballs

The human eye is particularly sensitive to the microgravity environment of the ISS. Over his year in space, Scott Kelly’s eyeballs was reshaped by fluid shifts caused by the low-gravity environment. His retinal nerve also thickened in response to microgravity.


This left Kelly with temporary vision problems, though he regained his pre-flight eyesight once he returned to Earth.

Spaceflight changes gene expression

The authors noted many contrasts between the twins’ epigenetic processes, which means that the two Kellys switched different genes on and off during the Twins Study. For instance, genes regulating Scott Kelly’s immune system were activated to acclimate to the strange ISS habitat, while Mark Kelly’s immune gene expression remained relatively static on Earth.

The paper also confirmed that spaceflight lengthened telomeres, which are caps on the ends of chromosomes, in Scott Kelly’s DNA. Longer lengths are associated with longer lifespans, so the ISS environment could be a venue for understanding aging processes, or even anti-aging technologies.

Scott Kelly’s gene expression mostly returned to normal within six months of his return to Earth, and his telomeres shortened to their average pre-flight length.

Long stays in space may cause cognitive decline

The Kelly twins achieved similar scores on cognitive tests before the spaceflight, as well as during the experiment’s early months. These exams included visual object learning tasks, which require memorization and recollection of shapes, and emotional recognition tests, which involve identifying basic emotions in images of faces.

During the latter half of Scott Kelly’s year in space and his first months back on Earth, his speed and accuracy in some of these tests declined.


“This extended mission duration may negatively affect cognitive performance postflight, which could have implications for safe mission operations (e.g., after a landing on Mars),” the team said.

More research is needed

Scott Kelly did not experience any significant permanent changes to his body due to his year in space, but that does not mean we’re ready to chuck humans over to Mars.

Read More: Why Scientists Are Surprised by Astronaut Scott Kelly’s ‘Space Genes’

By allowing their bodies to be sampled, observed, and tested for years, the Kelly twins have provided an amazing platform for understanding the challenges of spaceflight on human health. But it will take many more missions like the Twins Study to assess whether people can withstand the ravages of space travel over multi-year periods—especially when Earth is no longer visible just outside the spacecraft window.

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