What happens to your body after nearly a year in space? Plenty. Most things return to normal once you’re back on Earth ― but not all of them.

That’s the extremely topline summary of NASA’s “Twins Study,” the results of which were published Friday in the journal Science.

The study tracked the physiological and cognitive changes of astronaut Scott Kelly as he spent 340 days in space, then compared them to his twin brother, Mark Kelly ― who is also a retired astronaut ― back home.

NASA collected and analyzed 317 stool, blood and urine samples from both Mark and Scott over the course of the year. It also tested such things as the twins’ memory, as well as their heart and eye function. (Scott also received a flu vaccine aboard the International Space Station ― the first vaccine ever administered in space ― and his body reacted appropriately. While aboard the ISS, his immune system was much more active than his twin, the study found.)

Although 559 people have gone to space, only eight have spent more than 300 consecutive days in orbit. This has led to a scarcity of information about what happens to the human body in microgravity over time ― important information to have if NASA is serious about sending humans to Mars in the coming decades. A trip to the Red Planet takes about 300 days.

Scott returned to earth roughly three years ago, and he is mostly back to his regular self, NASA found, including his immune system, cognition and microbiome. But 7% of his gene expression has yet to revert to its pre-flight status, potentially because of DNA damage due to increased radiation exposure.