What happens when one twin goes to space and the other stays on Earth? Study explores effects of space travel.

Scott Kelly, left, and his twin brother, Mark Kelly, were the subject of a study as Scott spent a year in space and Mark remained on Earth.
Scott Kelly, left, and his twin brother, Mark Kelly, were the subject of a study as Scott spent a year in space and Mark remained on Earth. (Robert Markowitz / NASA)

Weight loss, altered eye shape and elongated DNA structures are just a few of the side effects of spending a year in space.

But after monitoring the differences between a pair of identical twins as one traveled to the International Space Station and the other stayed on Earth, researchers say there’s more work to be done to understand how long-term space travel can affect a person’s body.


A team of dozens of researchers released an article Thursday in the journal Science detailing the results of a study on Scott and Mark Kelly, twin brothers and astronauts. At age 50, Scott Kelly spent 340 days at the International Space Station, while Mark Kelly remained on Earth.

“Science is really important to our country and the science that we do in space impacts us for generations and generations,” Mark Kelly said in a conference call following the study’s release. “The stuff we learned from sending people to the moon in the 1960s and early ’70s have benefited our country for the last 50 years.”


Although researchers have a solid understanding of how shorter space missions affect the body, there is little research about long-term space travel impacts because only four people have participated in space missions lasting at least one year.

The study aimed to help researchers, including Johns Hopkins Hospital’s Dr. Andrew Feinberg, better understand the health impacts of spending long periods of time in space.

“When we got called into this study, we felt like part of this thing that’s bigger than ourselves,” Feinberg said.

The brothers were monitored for 25 months before, during and after Scott Kelly’s mission to the space station from March 27, 2015, to March 1, 2016. Researchers collected and compared results from blood, urine and stool samples throughout the study, and the brothers also took physical and cognitive tests.

The study found there were both temporary and lasting changes to Scott Kelly’s cells, tissues, genes and physical characteristics.

At least 10 key physiological processes were influenced by long-term space flight, according to the study: body mass and nutrition; the length of telomeres (features at the end of DNA strands); genome stability; vascular health; eye changes; metabolic changes; epigenetic shifts (changes to the expression of genes rather than the genetic code itself); changes to lipid levels; microbiome responses and cognitive function.

“It is expected that astronauts conducting exploration-class missions could experience risks from mitochondrial dysfunction, immunological stress, vascular changes and fluid shifts, and cognitive performance decline, as well as alterations in telomere length, gene regulation and genome integrity,” the study said.

Steven Platts, deputy chief scientist for NASA’s Human Research Program, said the new research reinforced conclusions about space travel that scientists had drawn already. The study will help inform future biomedical space research.

“Our bodies adapt and continue to function, and by and large continue to function very well,” he said.

Studying twins provided a unique opportunity for researchers to observe physical and genetic differences brought on by space travel. But because only two people were in the study, they said it’s impossible to conclude the variances between Mark and Scott Kelly were caused by space travel alone.

Feinberg, the Bloomberg Distinguished Professor of medicine, biomedical engineering and mental health at the Johns Hopkins University, studied changes in Scott Kelly’s epigenetics, which influence gene activity but not genetic code.

“This is the dawn of human genomics in space,” he said in a statement. “We developed the methods for doing these types of human genomic studies, and we should be doing more research to draw conclusions about what happens to humans in space.”


After Scott Kelly returned to Earth, most of the physiological changes he experienced in space reverted to their state before he went into orbit.

Understanding the biological impacts of long-term space travel will become increasingly important as NASA plots longer missions — particularly proposed trips to and from Mars that could last several years.

“Hopefully, our part in this study will help us get close to making a mission like that a success,” Mark Kelly said.

NASA plans to conduct future studies tracking the same markers for 10 astronauts on five more one-year missions. And although it took months for Scott Kelly’s body to readjust to Earth’s gravity after his return, he said he’d do it again.

“Put me in, coach,” he said. “I’m ready to go.”

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