Year In Space

NYT: "Scott Kelly Spent a Year in Orbit. His Body Is Not Quite the Same."

Compared to his twin brother who stayed on Earth:

DNA mutated in some of his cells. His immune system produced a host of new signals. His microbiome gained new species of bacteria. 

Many of these biological changes seemed harmless, disappearing after he returned to Earth. But others — including genetic mutations and, after his return, declines in cognitive test scores — did not correct themselves, provoking concern among scientists.