Space Time

A new study of astronaut twins — one who spent a record time in orbit, the other who stayed on Earth — offers a clearer understanding of how space travel affects the human body

Shannon Stirone

Apr 11, 2019·4 min read

In 2015, NASA astronaut Scott Kelly journeyed to the International Space Station (ISS), where he spent a year orbiting Earth. It was the longest single stretch of time any American had spent in space and gave scientists an opportunity to study how the body changes while in orbit. Even better, Scott Kelly has a twin brother, Mark Kelly, also an astronaut, who served as a scientific control on Earth, allowing researchers to precisely compare the two.

This was the most complex and in-depth study of human spaceflight to date. The average time any astronaut spends on the ISS typically maxes out at six months. In the months leading up to Scott Kelly’s yearlong flight, NASA scientists collected urine, fecal, and blood samples from both brothers. Scott also collected a cache of samples during his year in space. Once he returned, the medical analysis continued.

After years of analysis, more data is emerging to illuminate how Scott’s year in space changed his body. A new paper published in the journal Science reveals changes that occurred in Scott’s genes. Some were expected, while others were a surprise.

“The Twins Study is the most comprehensive view of the response of the human body to spaceflight ever conducted,” said Susan Bailey, one of the study’s authors and director of cancer biology at Colorado State University, over email.

We take for granted how safe it is to live on Earth. The planet’s atmosphere provides us with breathable air. Its magnetic field acts as a shield against the bulk of solar radiation and galactic cosmic rays. When a person leaves Earth’s protective boundary, almost all bets are off.

The microgravity environment of space allows our organs to shift around, which affects the way our heart muscles pump blood through the body. As a result, heart disease is a major risk for astronauts, and many are diagnosed with cardiovascular disease after returning to Earth. Other space explorers have developed irreversible eye damage because the muscles in their eyes changed with lower gravity, requiring them to use stronger glasses prescriptions in orbit and once they return to Earth.

All the data collected from the Twin Study is useful as the United States sets its sights toward long-duration spaceflight.

The new paper looks closely at the genetic changes that were previously noted among the Kelly brothers. Exposure to radiation in space can cause breaks in the strands of our DNA and jostle around chromosomes, causing them to swap places. This can force some genes to express themselves and others to stop expressing. These changes can increase the chances of cancer and other diseases. Approximately 7% of the genetic changes Scott Kelly experienced in space remained once he returned to Earth.

One change that particularly interested Bailey was Scott Kelly’s telomere data. Telomeres are structures at the end of our chromosomes that are considered biomarkers of aging. As we get older, our telomeres shorten. This is what researchers expected to see happen in Scott.

But instead of getting shorter, Scott’s telomeres unexpectedly grew. “We went into the study proposing that the unique stresses (nutritional, physical, psychological) and extreme environmental exposures (microgravity, space radiation) experienced by astronauts on long-duration missions would surely accelerate telomere shortening and thus increase the risk of aging and age-associated disease states like cardiovascular disease and cancer,” Bailey says. “What we observed were longer telomeres during spaceflight — exactly the opposite of what we thought.”

Once Scott Kelly returned to Earth, his telomeres quickly shortened and returned to preflight baseline levels. “However, he did have many more short telomeres after spaceflight than he did before,” Bailey says. “Our challenge now is to figure out the why and how of such dramatic shifts in telomere-length dynamics specifically associated with spaceflight.”

All the data collected from the Twin Study is useful as the United States sets its sights toward long-duration spaceflight. However, the study’s sample size is so small that far more data needs to be collected. We need more astronauts to spend longer than a year in space to really understand the effects of long-duration spaceflight on the human body.

NASA has stated its goals of building the next iteration of the ISS, called the Lunar Gateway, with the ultimate goal of returning to the moon. To do that — and to venture farther — we have to understand what’s safe for the astronauts we send into space. What level of risk would they be exposed to, and what would they be willing to undergo? After all, exploration doesn’t stop at the moon. NASA also wants to go to Mars, and that long journey — at least two years round-trip — will require a deeper understanding of the risks of human spaceflight.