Scientists have been trying for years to understand the effects of space on the human body, and they were presented with a great opportunity thanks to the fact that NASA astronaut Scott Kelly has a twin brother, Mark. So they decided to run some tests and compare Scott’s DNA to his brother’s after spending some time in space, and they were pretty surprised by the results.

Scientists found that Scott Kelly’s genetic makeup changed dramatically in the time he was on the International Space Station, with a total of 7 percent of genes remaining changed months after he returned to Earth. It’s a groundbreaking discovery that has huge implications for space travel, and for long-term colonies on places like the moon or Mars.

The research team measured the effects of oxygen deprivation, inflammation, nutrient changes, and of course the genes themselves to see how an extended stay in space affected things. While most genes returned to normal after he returned to Earth, 7 percent did not.

“Increasing mission duration from the typical six-month ISS mission to one year resulted in no significant decreases in Scott’s cognitive performance while inflight and relative to his twin brother Mark on the ground,” reads a NASA statement. “However, a more pronounced decrease in speed and accuracy was reported postflight, possibly due to re-exposure and adjustment to Earth’s gravity, and the busy schedule that enveloped Scott after his mission.”