After Living In Space For A Year, Identical Twin Comes Back Feeling Different

Can humans survive in space? NASA has been pondering that question for decades. In 2015, they conducted a study about this on two twin brothers. These identical twins are the only siblings in history to both be astronauts. When one twin lived in space for a year, he came back looking and feeling very different. Here are the scientific findings of NASA’s Twins Study.

Two Brothers, Twin Astronauts

Mark Kelly (L) and Scott Kelly attend Breitling Celebrates The North American Stopover.
Michael Stewart/WireImage
Michael Stewart/WireImage

On February 21, 1964, identical twins were born in Orange, New Jersey. They were Scott and Mark Kelly, and they would eventually become the only siblings in history to both travel into space.

Mark and Scott’s parents were both police officers. But the boys wanted to become pilots. In college, Mark got a master’s in aeronautical engineering, and Scott pursued naval aviation with a bachelor’s in electrical engineering. Both of them ended up in the same field.

Simultaneously, They Both Got Into NASA

Astronauts celebrate their graduation ceremony to join NASA.
MARK FELIX/AFP via Getty Images
MARK FELIX/AFP via Getty Images

In the 1980s, the brothers worked for the Navy. Both were pilots; Mark flew for over 50,000 hours in 50 different aircraft, and Scott flew over 40,000 miles in 40 aircraft. But in 1995, the twins decided to leave the Navy.

Scott and Mark both applied to NASA in 1995 to fulfill their childhood dreams of becoming astronauts. By 1996, they were admitted into the program. The brothers became Space Shuttle pilots and spent many years of their lives in space.

Would They Become A Science Experiment?

Two twin brothers, Scott and Mark Kelly, are twin astronauts.
ROBERT MARKOWITZ/NASA/AFP via Getty Images
ROBERT MARKOWITZ/NASA/AFP via Getty Images

When Scott and Mark joined NASA, they knew that they were ideal participants for scientific study. “Early on in our astronaut career, my brother and I had kind of wondered about it,” Scott Kelly said. “Hey, I wonder if they’ll ever do an experiment with the two of us, being genetically nearly identical.”

Because identical twins have the same DNA, it would be easy to track any changes in the body. If one twin spent more time in space, would it alter their health?

But For Decades, There Was No Interest

Astronauts Captain Scott Kelly and Captain Mark Kelly speak on stage at LocationWorld 2016.
Brian Ach/Getty Images for LocationWorld 2016
Brian Ach/Getty Images for LocationWorld 2016

The twins were happy to partake in an experiment. In fact, Scott Kelly brought up the subject to NASA shortly after he was hired. Researchers thought that using only two brothers was too small of a study to create anything noteworthy.

But in 2013, interest piqued again. Scott Kelly was scheduled to captain the International Space Station’s Expedition 45, which would remain in space longer than any other crew in history. This brought about a unique opportunity.

Why NASA Suddenly Became Interested

A NASA astronaut floats in space above the Earth.
Encyclopaedia Britannica/UIG Via Getty Images
Encyclopaedia Britannica/UIG Via Getty Images

Why did NASA suddenly get interested in 2013 and not in the 1990s? According to Business Insider, NASA is aiming to put humans on Mars in the 2030s. Although we have the rocket technology to do so, scientists still need more information about the human body.

Humans have not remained in space long enough to determine the health effects. Will they have compromised immune systems? Can they still live as long? Before putting humans on Mars, scientists need to answer those questions.

Scott Kelly Became The Space Subject

Captain Scott Kelly sits in his space suit outside the Soyuz TMA-18M spacecraft.
Bill Ingalls/NASA via Getty Images
Bill Ingalls/NASA via Getty Images

In 2013, Scott Kelly signed up to live in space for a full year. He captained a crew alongside Russian cosmonaut Mikhail Kornienko. “When it came to the fact that I was going to spend a year in space, it was so unique that I actually thought maybe there was some merit to it,” Scott said.

The discussion soon became a study on the Kelly twins. “It turns out there was some interest once people started talking about it,” Scott added.

Scott Spent A Full Year Up In Space

Scott takes a photo of him in the International Space Station.
NASA via Getty Images
NASA via Getty Images

On March 27, 2015, Scott Kelly and Mikhail Kornienko flew up to the International Space Station (ISS), which is a large spacecraft that orbits the Earth. They spent a record-breaking amount of time in orbit. It was called the One-Year Mission.

Usually, astronauts remain in space for around 182 days. But Kelly and Kornienko lived in space for 340 days, the longest any human has ever stayed. For a Mars round-trip, humans would be in space for around three years.

During This Trip, He Tested Himself

Scott Kelly wears special glasses while in the International Space Station in space.
NASA via Getty Images
NASA via Getty Images

While Scott Kelly was up in space, he conducted several tests on himself and sent the results back to earth. He recorded everything, including urine samples, blood tests, cognitive quizzes, saliva samples, fecal matter, and more.

In addition, the spacecraft’s environment was closely monitored. Astronauts understand that the craft’s radioactivity and lack of gravity can affect the body. Like all astronauts, Scott adhered to a strict and busy schedule. This amount of stress can also impact the body physiologically.

Meanwhile, Mark Remained On Earth

Mark Kelly sits and ponders next to a statue of a rocket.
Eric Kayne/Getty Images
Eric Kayne/Getty Images

As Scott Kelly spent the year up in space, Mark Kelly remained on Earth. But he was not on vacation. Mark performed all of the tests that Scott did: blood tests, urine samples, tracking diet and work, etc. Everything in his life was closely monitored.

Even so, Mark’s life was more carefree than Scott’s. While Mark could eat whatever he wanted, Scott stuck to a strict diet and work schedule. Being 250 miles away from each other, they lived different lives.

Even When Scott Landed, Testing Continued

Scott Kelly exits the rocket after his space expedition.
Joel Kowsky/NASA via Getty Images
Joel Kowsky/NASA via Getty Images

Scott Kelly returned to Earth on March 1, 2016. But testing did not end there. Ten teams of scientists continued to analyze his body for six more months. They wanted to determine if returning to Earth made the human body go back to normal.

For example, astronauts lose muscle mass because of the lack of gravity. When they return to Earth, they can exercise to recover muscle. Does the same occur for the immune system or gut health?

The Research Was Not Released Until A Year Later

A rocket arrives at the International Space Station.
NASA via Getty Images
NASA via Getty Images

The Twin Study required 84 researchers from 12 different universities and almost four years of study. The teams examined every part of the Kelly brothers, from DNA to genes to inflammation. On April 11, 2019, the study was finally published in the journal Science.

“This is really probably the most in-depth study, certainly at the biochemical level, that’s ever been done in people in space,” said the study’s co-author, Mike Snyder. Researchers compared Scott’s genetics to Mark’s to learn how humans adapt to space.

Scott’s Genes Changed In Space

Scott Kelly sits inside of a rocket in preparation to go to space.
Bill Ingalls/NASA via Getty Images
Bill Ingalls/NASA via Getty Images

The most notable change in Scott Kelly was in his genes. Christopher Mason, a geneticist at Weill Cornell Medical Center, said that over 1,000 of Scott’s genes changed while in space. Part of Scott’s chromosomes were inverted, and some of them flipped.

“The most enriched set of genes [were] almost all involved in the immune system regulation,” Mason explained. “The immune system is almost on a high alert as a way to try and understand this new environment.”

Oddly, Mark Had More Genetic Changes

Mark Kelly (right) stands next to his twin brother, Scott Kelly.
Bill Ingalls/NASA via Getty Images
Bill Ingalls/NASA via Getty Images

While Scott was in space, his DNA changed around six times more during the second half of his trip. But Mark’s genetics also changed–even more than Scott’s! Their DNA simply adapted in different ways.

Andrew Feinberg, a geneticist at Johns Hopkins University, believes that this is due to stress. With more travel and a constantly-changing diet, Mark had a more chaotic life on Earth. Scott experienced a different kind of stress in space, which might have altered his body.

Scott’s Chromosomes Became Longer

A close-up photo shows human telomeres on the ends of chromosomes.
Smith Collection/Gado/Getty Images
Smith Collection/Gado/Getty Images

Most notably, Scott’s chromosomes grew while in space. His telomeres, the ends of chromosomes that protect DNA, lengthened. This is the opposite of what scientists expected.

Telomeres control a person’s aging. Throughout a person’s life, telomeres become shorter as they age. Susan Bailey, a professor of radiation at Colorado State University, believes that an enzyme called telomerase activated while Scott was in space. This enzyme rapidly repaired his chromosomes from the radiation, which possibly lengthened the telomeres.

Can People In Space Live Longer?

Scott Kelly takes a picture in his space suit before liftoff.
NASA via Getty Images
NASA via Getty Images

Scott’s telomere markers brought up the question: will people in space live longer? It is not likely, says Andrew Feinberg, a co-author of the study. Within 48 hours of landing, Scott’s telomeres returned to their normal length.

“I don’t think that [the elongation] can really be viewed as the fountain of youth and that people might expect to live longer because they’re in space,” said Bailey. In fact, the unusual lengthening and shortening of Scott’s telomeres might have shortened his life.

Scott Even Looked Different Afterward

The Kelly twins face each other.
ROBERT MARKOWITZ/NASA/AFP via Getty Images
ROBERT MARKOWITZ/NASA/AFP via Getty Images

When Mark saw his brother for the first time in a year, he looked mostly the same. However, his eye shape was slightly different. His eyeballs were altered, and his vision was not as sharp. According to the study, this might have resulted from carbon dioxide exposure.

At a minimum, space has ten times more carbon dioxide than Earth. “I always felt pressure in my head,” Scott Kelly recalled. “It would burn your eyes. I was able to tell what the CO2 level was pretty accurately without having to look at the measurement.”

His Had To Adjust To A Different Blood Flow

Astronaut Scott Kelly floats outside of the ISS.
NASA via Getty Images
NASA via Getty Images

On Earth, gravity pushes most blood flow down to your legs. But without gravity, the brain and chest receive more blood. Scott experience more fluid flow into his head than his Earthbound brother, Mark.

“Once you get back to Earth, all the blood just wants to pool in your legs,” Scott said during an interview. When he got back, Scott felt pain in his legs, and his skin got rashes and hives whenever he put pressure on it. Eventually, these symptoms went away.

Scott’s Body “Restructured”

Astronaut Scott Kelly interviews with the media after returning from one year in space.
Eric Kayne/Getty Images
Eric Kayne/Getty Images

A research team from the University of California, San Diego, examined how Scott’s proteins changed. Not only did his eyes change, but his circulatory system also changed. When he had more blood flow near the head, his arteries and blood vessels had hardened.

“It just kind of shows how resilient the body is, and how adaptive the human body is to different environments,” explained team leader Brinda Rana. “A year in space—the body can handle that.” Perhaps it can handle three years in the future.

And His Body Produced Different Proteins, Too

Scott Kelly climbs stairs to enter the Soyuz TMA-16M spacecraft.
Bill Ingalls/NASA via Getty Images
Bill Ingalls/NASA via Getty Images

The San Diego research team noted that Scott Kelly’s body had “restructured.” While in space, his body produced different proteins. It created more collagen, a protein that builds hair and fingernails. Scott also had more aquaporin 2, a marker of dehydration.

Brinda Rana, the health sciences researcher who led the team, said that Scott’s body was adjusting to space travel. Once Scott returned to Earth, these symptoms disappeared, along with many other findings from this study.

Ninety Percent Of Scott’s Genes Were The Same

Scott Kelly walks toward a space shuttle with his crew.
Philip Scott Andrews/Roll Call/Getty Images
Philip Scott Andrews/Roll Call/Getty Images

When Scott returned from his trip, scientists were eager to learn how his genomes had changed. Stressful situations can alter human genes, as can the radiation and isolation in space. But Scott’s genomes were “not worrisome,” claimed medical researcher Andrew Feinberg.

Scott had some genetic differences; but after landing, 90% of his genes returned to normal. Mark and Scott both had few genome changes. According to Feinberg, this proves that human genes can return to normal after a long space mission.

What About The Other 10%?

A DNA film projects the codes of a person's DNA.
KIM KULISH/AFP via Getty Images
KIM KULISH/AFP via Getty Images

But 10% of Scott’s genes–around 800 of them–did not return to normal right away. These genes mainly controlled the immune response and DNA repair. They were still working overtime to repair Scott’s body.

“It seems, to some degree, that enough cells in the body have a memory of what happened that there’s still some ongoing adaptation and recalibration to being back on Earth,” explains Chris Mason. In other words, Scott’s time in space might have left some trauma on his body.

Scott Might Have A Higher Risk Of Age-Related Diseases

Scott Kelly takes a selfie in the rocket before he launches into space.
NASA via Getty Images
NASA via Getty Images

Scott’s lengthening and shortening telomeres might have had unexpected consequences. Bailey believes that this might have put too much strain on the body; it could even heighten Scott’s risk of age-related cardiovascular diseases and cancer.

But researchers need more time to confirm this. According to the NASA website, “long-term effects of this research will be studied for years to come.” Although the twins have stopped taking blood samples, they will continue to be the subjects of research.

The Most Worrisome Change Was Psychological

Scott Kelly of NASA (left) and Mikhail Kornienko of Roscosmos (right) celebrate their 300th consecutive day in space with a sign.
NASA via Getty Images
NASA via Getty Images

Throughout his trip, Scott took frequent cognitive tests that featured puzzles and computerized tests. But his results rapidly declined during the second half of the flight. While his scores were not alarming, they continued to decline after he landed.

Scott’s cognitive test results were lower, and he took longer to answer, said psychiatry professor Mathias Basner. When planning a trip to Mars, these results are concerning. Emotional regulation, risk-taking, and close attention are necessary for space travel.

Why Scott’s Cognition Declined

Three men help Scott Kelly get out of a space capsule.
KIRILL KUDRYAVTSEV/AFP via Getty Images
KIRILL KUDRYAVTSEV/AFP via Getty Images

Basner’s team at the University of Pennsylvania proposed that Scott’s cognitive decline resulted from stress. “Space is a very hostile environment,” Basner explained. “Tiny mistakes can translate into catastrophic errors—in the worst case, to mission failure and loss of equipment and of astronaut lives.”

Scott’s test scores might also result from post-flight pain, tiredness, or a lack of motivation. But throughout the flight, his scores became less accurate. Astronauts on a three-year trip to Mars might struggle with psychological stress.

His Gut Bacteria Changed, Too

In this International Space Station, Scott Kelly juggles fresh fruit that arrived for his crew.
NASA via Getty Images
NASA via Getty Images

While the scientists were studying Scott’s behavior, a research team at Northwestern University analyzed his gut microbiome. Different types of bacteria increased while in space, while others decreased, the researchers found. However, the diversity of gut bacteria remained the same, which proves that the intestines can still be healthy in space.

But all of that was temporary. After Scott landed, his microbiome returned to normal. Geneticist Martha Hotz Vitaterna thinks that his changing gut bacteria resulted from a variance in his genes.

The Possible Long-Term Health Effects

A doctor speaks to a patient as a sphygmomanometer, or blood pressure meter, lies on his desk.
Adam Berry/Getty Images
Adam Berry/Getty Images

Although it is too soon to determine the long-term effects of space travel, researchers have predicted a few things. For one, Scott’s arteries hardened from a higher blood flow, and his heart weakened. These are both markers of potential heart disease.

Scott believes that his great symptom might be psychological. “You’ve experienced this significant event where you’re living in a very controlled environment for a really long time, and then you don’t have that anymore,” he said while describing space travel.

Scott Also Took Longer To Recover

Scott Kelly rubs his eye as his space suit is being tested.
KIRILL KUDRYAVTSEV/AFP via Getty Images
KIRILL KUDRYAVTSEV/AFP via Getty Images

Scott’s One-Year Mission was not his only space flight. “[I had] flown flights of increasing duration throughout my career: seven days; 13; 159 and 340,” Scott told Space.com. But the longer he remained in space, the longer he took to recover back home.

“I was in space more than twice as long [as last time] — I felt more than twice as bad when I got back,” he continued. He spent about eight months in recovery. Despite this, Scott believes that astronauts can handle a three-year trip to Mars.

It’s Too Early To Generalize The Results

A patch portrays the logo for the NASA Twins Study.
RJ Sangosti/The Denver Post via Getty Images
RJ Sangosti/The Denver Post via Getty Images

Although the Twins Study has provided some important insights, it is too early to apply all of these results to astronauts. Researchers at the University of California, San Diego, emphasized that the sample size is too small.

Even so, Mark and Scott Kelly provided a “valuable roadmap” for further research, the scientists said. NASA now knows what to expect from long space travel. And more importantly, they know that astronauts can handle many years in space.

Scott Kelly Believes That People Can Go To Mars

Scott Kelly stands next to a model of the planet Mars.
Eduardo Parra/Getty Images
Eduardo Parra/Getty Images

Despite Scott’s struggles after being in space for a year, he truly believes that humans can go to Mars. “The bottom line is, from all these studies…there’s nothing that we saw that would prevent us from going to Mars,” he told Space.com.

“Certainly, there’s some stuff that they’re going to continue to look at — gene expression, telomeres, other issues astronauts have with their vision,” he continued. But the study revealed “no showstoppers” that would prevent humans from heading to Mars.

NASA Will Continue With Future Experiments

The International Space Station is seen above the Earth.
Nasa/Getty Images
Nasa/Getty Images

Because of the success of the Twins Experiment, NASA has planned other studies with future astronauts. According to a Colorado State University press release, NASA has scheduled “ten astronauts on one-year missions, ten on six-month missions, and ten on trips from two to three months at a time.”

Just like the Twins Experiment, these studies will track other astronauts who remain on Earth. Researchers will also analyze the amount of bed rest required when the astronauts return home.

Future Space Travel Might Require Artificial Gravity

Five men and women walk along Mars in a storm.
Francis DEMANGE/Gamma-Rapho via Getty Images
Francis DEMANGE/Gamma-Rapho via Getty Images

During an interview with Scientific American, Scott Kelly said that, if humans are to go beyond Mars, we will need to experiment with artificial gravity. Artificial gravity is used to train astronauts for extreme conditions in space.

“I flew in space for seven, 13, 154 and then 340 days,” Scott said. “The longer you’re there, the more symptomatic you are when you return. I couldn’t imagine coming back to Earth after being in space for many years.”

Where Are The Kelly Twins Now?

Scott Kelly and Mark Kelly stand together to accept NASA awards.
Tim Mosenfelder/Getty Images
Tim Mosenfelder/Getty Images

Scott Kelly retired from NASA in 2016. That same year, he was appointed as a United Nations Champion for Space. He also wrote a novel about his space life, called Endurance: A Year in Space, a Lifetime of Discovery.

Mark Kelly retired earlier in 2011. He has written several books, founded the nonprofit organization Americans for Responsible Solutions, and co-founded the near-space exploration company World View Enterprises. In 2020, he became the Senator of Arizona. Both brothers will continue to work with NASA researchers for the rest of their lives.