Astronaut Scott Kelly and His Twin Brother Are Still Identical, NASA Says

Scott and Mark Kelly
Twin astronauts Scott and Mark Kelly pose at Johnson Space Center in Houston on Jan. 19, 2015, before Scott Kelly's nearly yearlong stay on the International Space Station. (Image credit: Robert Markowitz)

After a stream of erroneous media coverage about how spaceflight affects astronauts' genes, NASA issued an updated statement yesterday (March 15) about its "twins study" of former astronauts Scott and Mark Kelly.

The study is following changes to Scott Kelly's body after he spent nearly a year in space between 2015 and 2016. His brother and identical twin Mark remained on Earth during that time and is the control subject for the study. In late January, NASA issued an update to its 2017 results that confirmed most of the initial findings.

"Mark and Scott Kelly are still identical twins; Scott's DNA did not fundamentally change. What researchers did observe are changes in gene expression, which is how your body reacts to your environment. This likely is within the range for humans under stress, such as mountain climbing or scuba diving," NASA said in the recent clarification to the Jan. 31 update. [Twins In Space: Astronauts Mark and Scott Kelly in Photos]

NASA's update came after some media outlets initially misreported that Scott Kelly's DNA itself had changed.

"The change related to only 7 percent of the gene expression that changed during spaceflight that had not returned to preflight [levels] after six months on Earth," NASA officials wrote. "This change of gene expression is very minimal. We are at the beginning of our understanding of how spaceflight affects the molecular level of the human body. NASA and the other researchers collaborating on these studies expect to announce more comprehensive results on the twins studies this summer."

The brothers joked about the media coverage on their Twitter accounts.

"What? My DNA changed by 7%! Who knew? I just learned about it in this article," wrote Scott Kelly, who linked to a Newsweek article in a tweet on March 10. "This could be good news! I no longer have to call @ShuttleCDRKelly my identical twin brother anymore." 

Mark Kelly added his input yesterday (March 15) while linking to a CNN article. "I used to have an identical twin brother. Then this happened," he joked. After he tweeted, the CNN article was updated. 

Several reporters also wrote articles pointing out the erroneous information spread by other news outlets. 

"The NASA result everyone is freaking out about actually measured Scott Kelly's expression levels, and it found that — not surprisingly — spaceflight affects how much expressing certain genes do, particularly those involved in immune function, DNA repair pathways, and bone growth," Nadia Drake wrote in National Geographic.

"Kelly's base DNA didn't actually change by seven percent during his time in space. His gene expression — the transcribing and translation of genes, not the genes themselves — was what actually changed during his year on the space station," added Miriam Kramer in Mashable.

Ars Technica's John Timmer's roundup of coverage pointed out errors in several articles, including in a now-corrected story by's sister site Live Science. For that story, he pointed out problems not only with the description of DNA altering but with a phrase saying Kelly's genetic code had changed. Timmer said changing a person's genetic code would actually kill them. (Live Science posted a follow-up piece today.)

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: