Scott Kelly Returned 5 Milliseconds Younger Than His Twin After 340 Days on ISS

1 articles · Updated · spacedaily.com · May 18

NASA calculated that Scott Kelly came back from a 340-day ISS mission about 5 milliseconds younger than his identical twin Mark after landing in March 2016.
The gap came from special relativity: the station circles Earth at about 17,500 mph, and that speed slows onboard clocks enough to outweigh the slightly faster ticking caused by weaker gravity 260 miles up.
The effect is tiny but measurable with precise clocks, and it is still accruing for the seven astronauts now aboard the ISS, which completes an orbit every 90 minutes.
Kelly’s twin study also tracked broader health changes in orbit, including longer telomeres that later reverted, shifts in gene expression and the gut microbiome, and a slight post-flight cognitive slowdown.
Scott Kelly’s 5 milliseconds is not the human record: Russian cosmonaut Sergei Krikalev, with more than 803 days in orbit, is estimated to be about 20 milliseconds younger than if he had stayed on Earth.

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