They called it the "perfect nature versus nurture" experiment: Astronaut Scott Kelly was sent to the International Space Station for nearly a year, while his identical twin, astronaut and now Sen. Mark Kelly, remained on Earth. When Scott Kelly came back home in March 2016, scientists compared his DNA and body makeup to that of his twin and found Scott was 2 inches taller than he'd been and boasted a lower body mass, among other changes (most of which have since reverted). Now, per a study published Monday in the journal Circulation, another big shift announced: The mass of Kelly's heart shrank during his cosmos travels from 6.7 ounces to 4.9 ounces, or about 27%, though the astronaut didn't seem to suffer any major repercussions because of it. "He did remarkably well over one year," study co-author Benjamin D. Levine, an internal medicine professor at the University of Texas Southwestern Medical Center, tells the New York Times.
CNN's explainer: When humans are dealing with Earth's gravity, the heart has to work harder to pump blood. In zero gravity, however, the heart doesn't have to work so hard and thus atrophies. Levine tells the Times that Kelly's heart did so without becoming "dysfunctional." The study also looked at Benoit Lecomte, an endurance swimmer who tried to cross the Pacific in 2018. The water's buoyancy and Lecomte's near-constant horizontal state during his five-month-plus swim produced conditions similar to Kelly's weightlessness, and his heart reduced in size nearly as quickly. Although Kelly ended up fine, he also worked out quite a bit while in space using exercise gear—likely keeping his heart from weakening any further. Scientists' concern is that astronauts on future missions could be prevented from exercising due to injury, illness, or broken equipment and suffer more serious effects. NASA is funding further research on the heart involving astronauts on both long- and short-haul trips. (Read more discoveries stories.)