--- title: Astronaut Twin Study Shows Possible Stress of Space Missions description: Early results of an unprecedented study to determine the effects of space travel show genetic changes in astronaut Scott Kelly. canonical: https://time.com/4651309/scott-kelly-mark-kelly-twin-study/ author: Melissa Chan article:opinion: false article:content_tier: free article:published_time: 2017-01-27T01:03:55.000Z article:modified_time: 2026-06-24T06:56:54.495Z article:section: Science og:title: Astronaut Twin Study Shows Possible Stress of Long Space Missions og:description: There were changes in Scott Kelly’s gene expression og:url: https://time.com/4651309/scott-kelly-mark-kelly-twin-study/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt58ddc7d0b3210695/6988b7838e06018bc2468f9e/scott-kelly-mark-kelly.jpg?branch=production&width=2400&quality=75&auto=webp&crop=16:9 og:image:width: 1200 og:image:height: 675 og:image:alt: scott-kelly-mark-kelly og:type: article twitter:card: summary_large_image twitter:title: Astronaut Twin Study Shows Possible Stress of Long Space Missions twitter:description: There were changes in Scott Kelly’s gene expression twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/blt58ddc7d0b3210695/6988b7838e06018bc2468f9e/scott-kelly-mark-kelly.jpg?branch=production&width=2400&quality=75&auto=webp&crop=16:9 ---  # Astronaut Twin Study Shows Possible Stress of Long Space Missions ## Video: This Video Is No Longer Available [Watch (HLS stream): This Video Is No Longer Available](https://cdn.jwplayer.com/manifests/fKodeUO6.m3u8) (0:10)  _Published 2026-06-23. There were changes in Scott Kelly’s gene expression_  by [Melissa Chan](https://time.com/author/melissa-chan/)  ## Melissa Chan Jan 27, 2017 1:03 AM UTC  by [Melissa Chan](https://time.com/author/melissa-chan/)  ## Melissa Chan Jan 27, 2017 1:03 AM UTC [Early results](http://www.nature.com/news/astronaut-twin-study-hints-at-stress-of-space-travel-1.21380) of an unprecedented study to determine the effects of space travel show genetic differences between [astronaut Scott Kelly](http://time.com/4256439/scott-kelly-retirement-nasa/), who spent a year in space, and his identical twin brother who stayed on Earth. There were changes in Kelly’s gene expression and DNA methylation, meaning the genes were essentially chemically activated, according to preliminary results published Thursday on the journal _Nature_’s website. “Almost everyone is reporting that we see differences,” said Christopher Mason, a geneticist at New York’s Weill Cornell Medical College, according to Nature. It’s unclear what the preliminary findings indicate. The brothers will be closely followed for at least four more years. Kelly and his brother Mark Kelly, who served as the test’s control, agreed to help scientists learn more about the effects of space on one’s body. The latest results were presented at the annual meeting of the Human Research Program in Galveston Kelly spent 340 days in space before returning home in March 2016\. TIME covered the Kelly brothers’ mission extensively in an Emmy-nominated 12-episode series, which can be viewed at [time.com/space](http://time.com/space-nasa-scott-kelly-mission/). ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/"},"headline":"Astronaut Twin Study Shows Possible Stress of Long Space Missions","datePublished":"2017-01-27T01:03:55.000Z","dateModified":"2026-06-24T06:56:54.495Z","description":"There were changes in Scott Kelly’s gene expression","url":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/","keywords":["Space","onetime"],"thumbnailUrl":"https://static.time.com/v3/assets/bltea6093859af6183b/blt58ddc7d0b3210695/6988b7838e06018bc2468f9e/scott-kelly-mark-kelly.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","author":[{"@type":"Person","name":"Melissa Chan","jobTitle":null,"url":"https://time.com/author/melissa-chan/"}],"articleSection":"Science","image":[{"@type":"ImageObject","url":"https://static.time.com/v3/assets/bltea6093859af6183b/blt58ddc7d0b3210695/6988b7838e06018bc2468f9e/scott-kelly-mark-kelly.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","width":1200,"height":675,"headline":"scott-kelly-mark-kelly","caption":"Scott Kelly and Mark Kelly are pictured here on Nov. 2, 2016 in New York City.","creditText":"Brian Ach—2016 Getty Images","representativeOfPage":true}],"publisher":{"@type":"Organization","name":"Time","url":"https://time.com/","logo":{"@type":"ImageObject","url":"https://time.com/images/logo.png","width":528,"height":156},"foundingDate":"March 3, 1923","sameAs":["https://www.facebook.com/time","https://www.instagram.com/time/?hl=en","https://twitter.com/time","https://www.pinterest.com/timemagazine"]}},{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/section/science/","name":"Science"}},{"@type":"ListItem","position":2,"item":{"@id":"/tag/space/","name":"Space"}},{"@type":"ListItem","position":3,"item":{"@id":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/","name":"Astronaut Twin Study Shows Possible Stress of Long Space Missions"}}]},{"@context":"https://schema.org","@type":"VideoObject","name":"This Video Is No Longer Available","description":"There were changes in Scott Kelly’s gene expression","thumbnailUrl":"https://cdn.jwplayer.com/v2/media/fKodeUO6/poster.jpg?width=720","uploadDate":"2026-06-23T15:00:44.000Z","contentUrl":"https://cdn.jwplayer.com/manifests/fKodeUO6.m3u8","embedUrl":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/","duration":"PT10S","potentialAction":{"@type":"SeekToAction","target":"https://time.com/4651309/scott-kelly-mark-kelly-twin-study/?jw_start={seek_to_second_number}","startOffset-input":"required name=seek_to_second_number"}}] ```