Only the brain seems to love it; after all, it’s the one that fervently processes the beautiful views of the gleaming planet below, delights in the somersaults made natural by microgravity, and comprehends—or at least attempts to comprehend—the wonder of being there, in outer space.

Read: What’s different about astronaut DNA?

Scientists had expected some of the changes observed in Scott, based on earlier research of astronauts. Some of the effects were no more dramatic than stress-related changes studied on Earth—and space travel is certainly stressful. The others, such as the eyeball squishing, can clearly be attributed to Scott’s unique experience in space.

But that’s where the explanations end. With all the variables involved, isolating a single cause is nearly impossible. Researchers can’t know whether the changes were caused by microgravity, increased exposure to radiation, lack of air circulation, sleep disruption, a diet of freeze-dried food, or the stress of living in a cramped metal tube with the same people, day in and day out.

Some of the most intriguing changes occurred at the chromosomal level, in the protective bits at the ends of chromosomes that make sure they replicate properly when cells divide. These caps, known as telomeres, are known to shorten as a result of stress. Researchers expected to see this change in Scott. Instead, the astronaut’s telomeres lengthened. “You might at first think, Oh, this is great. He’s going to live longer,” Susan Bailey, the Colorado State University professor who led the telomere research, once told me. “But the opposite side of that coin is always that it also increases cancer risk, because one of the very first things cancers do is turn telomerase on to maintain telomere length so they can essentially be immortal.”

Most of the telomeres bounced back after Scott returned to Earth, but he now has more short telomeres than he did before his mission. In general, this puts someone at greater risk for quicker aging, Bailey said.

Researchers found some surprises in Scott’s gene expression. On Earth, changes in gene behavior occur in response to shifts in routine activities, such as sleep and diet, and Mark’s gene expression changed as well. But the changes to Scott’s gene expression were distinct, and scientists were stunned at the number of changes they recorded, especially in mitochondrial genes, which help the body produce energy, and in genes related to the immune system. More than 90 percent of these genes returned to normal when Scott came back. (This doesn’t mean, researchers are careful to note, that the rest are somehow “mutated,” as some news reports erroneously suggested last year.)

Researchers also detected changes in the mechanism that cells use to control gene expression, but they were too tiny to matter by the time Scott came back.