Penn Medicine’s Mathias Basner discusses the NASA Twins Study, which analyzed astronaut Scott Kelly’s physical and mental health after he spent 340 days in space, and found that Kelly’s performance on a cognitive test battery dropped when he returned to Earth for six months.
Astronaut Scott Kelly on his nearly year-long mission on the International Space Station. (Photo: NASA)
No one really knew how Scott Kelly’s body might change after spending almost a year in space. Though the scientific community had some ideas, previous astronauts rarely spent more than six months in microgravity, and none had an identical twin who was also an astronaut.
Now, three years after Kelly returned from his 340-day mission, the NASA Twins Study has yielded some answers, with the findings published recently in the journal Science. Penn Medicine’s Mathias Basner, an associate professor in psychiatry, led the team that studied the cognitive performance of Kelly and his twin brother, Mark.
“The 10 investigations spanned a wide range of outcomes, including gene expression, the gut microbiome, and the immune response to vaccination. We were looking at Scott’s behavioral health,” Basner says. “Most of the other studies concentrated on the clockwork, but we were investigating the movement of the dials.”
Penn Today spoke with Basner about how his team joined the NASA study, what they found, and what it means for future—and likely longer—spaceflight missions.
Identical twin astronauts, Scott Kelly (right) and Mark Kelly—both now retired—participated in what’s become known as the NASA Twins Study. Scott spent 340 days in space while Mark acted as a control on Earth. (Photo: NASA)
How did you get involved in the NASA work?
About eight years ago, we started developing a cognitive test battery for NASA that was tailored for astronauts. This set of tests looks at a range of cognitive domains important for mission success in spaceflight and is much broader than what NASA was using at the time.
Astronauts are high performers and they’re highly motivated, so taking an off-the-shelf battery of cognitive tests doesn’t work well for them. The tests would be too easy and would not allow us to detect subtle spaceflight effects. Our colleague, Penn neuropsychologist Ruben Gur, already had a well-validated computerized battery of cognitive tests. We combined his with a test developed by David Dinges, also here at Penn, then shortened and tailored the tests for the astronaut population. NASA selected us as one of 10 teams to monitor the twins before, during, and after Scott’s historic one-year mission on the International Space Station (ISS).
Can you describe some of these cognitive tests?
Each test probes a specific cognitive domain like memory, attention, reasoning, or spatial orientation, domains that are all critically important for successful space missions. One test, for example, investigates emotion recognition. The astronaut looks at faces, then has to select the correct emotion—happiness, sadness, fear, anger, or no emotion—displayed on a screen. During a mission to Mars, we want the crew to be able to read each other’s faces, to promote team cohesion and avoid conflicts. Another test concentrates on risk decision making. It requires astronauts to pump up balloons as far as possible, but to stop pumping before the balloons pop. It’s important that astronauts take the right amount of risk in space, since small mistakes can have catastrophic consequences.
The recent paper revealed some fascinating insights, like the fact that Kelly’s telomeres, the DNA stretches found at the end of chromosomes, actually lengthened rather than shortened as tends to happen with age. What were your team’s biggest findings?
The main question we were trying to answer was whether prolonging a mission from the standard six months to a year would affect Scott’s performance on our cognitive tests. The good news is that we didn’t find major differences between Scott’s first and second six months on the ISS. This suggests that astronauts can cope with living in space for a year.
The surprising finding was that, once Scott returned to Earth, we saw a more relevant decline in performance across almost all 10 tests; he was slower and less accurate, and this effect persisted for six months after he returned, when we performed our final test. This matches Scott’s own perception that he found it harder to adjust to Earth’s gravity after this longer mission compared to an earlier, shorter one.
You mentioned a big caveat is that these findings are based on the experience and biological data of just one person, what’s referred to as an n of 1. What can we infer from this?
First, it means that these findings don’t necessarily generalize to other astronauts. Also, we don’t know how the performance on our tests translates to operational performance. Astronauts are highly trained, which means they can compensate to a certain degree in the face of adversity. Regardless, we always want astronauts to perform at their best, especially during mission-critical phases, like when they descend to the surface of Mars and establish a base camp there, for instance. Our results suggest that re-exposure to gravity could pose some problems, despite Mars only having a third of Earth’s gravity. We need to investigate this further.
How much closer do these findings bring us to understanding how the human body reacts to prolonged time in microgravity?
This was an important first step toward safely sending and returning astronauts to and from Mars. With this study, NASA managed to establish the policies required for performing biomedical space research in astronauts on future space missions, studies that will ideally reveal the risks inherent to long-duration exploration-type spaceflight.
We feel privileged that NASA selected us to be part of 10 more one-year ISS missions that will likely start in 2021. Beyond cognitive testing, we will perform structural and functional neuroimaging in astronauts before and after the flight to determine the biological basis of any changes we observe in cognition. The study will also include 10 six-month ISS stays and 10 two-month ISS stays, so we can hopefully build a link between mission duration and the cognitive effects we observe.
Funding for the work came from NASA (Grant NNX14AH27G) and the National Space Biomedical Research Institute (Grant NASA NCC 9-58).
The Polyhedral Structures Laboratory is housed at the Pennovation Center and brings together designers, engineers, and computer scientists to reimagine the built world. Using graphic statics, a method where forces are mapped as lines, they design forms that balance compression and tension. These result in structures that use far fewer materials while remaining strong and efficient.
From ancient tombs and tiny robots to personalized gene editing and AI weather models, Penn’s 2025 research portfolio showed how curiosity—paired with collaboration—moves knowledge into impact and stretches across disciplines and continents.
Centering joy in AI development and implementation
PIK Professor Desmond Upton Patton—of Annenberg and SP2—and collaborators introduce a joy-informed framework designed to initiate conversations among engineers, designers, and researchers.
Reflecting on Jane Austen, 250 years after her birth
English professors Michael Gamer and Barri Joyce Gold have been teaching courses specifically dedicated to Jane Austen for years. They spoke with Penn Today about their approach to teaching her novels, how they challenge common readings and myths, and what makes Austen’s work so enduring—and adaptable to the screen—more than two centuries later.
Electronic medical records help save lives of HIV patients
Wharton’s Leandro “Leo” Pongeluppe and colleagues found that HIV clinics in Malawi that switched from paper to electronic medical records saw an estimated 28% reduction in deaths in five years.
