What happens to the human body in space?

Carl Zimmer, "Scott Kelly Spent a Year in Orbit. His Body Is Not Quite the Same." The NYtimes, April 11, 2019.

Scott Kelly spent 340 days on the International Space station while his twin brother, Mark, stayed on the ground. "He drew blood from his arms. He saved his urine. He played computer games to test his memory and reaction speed. He measured the shape of his eyes." And his brother did the same. A comparison of the data indicates a large number of differences between then two:

On Thursday, just over three years after Mr. Kelly, 55, returned to Earth, NASA researchers reported that his body experienced a vast number of changes while in orbit. 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.

Some considered the risks manageable, while others wondered whether it would ever be safe for astronauts to take long journeys to Mars or beyond. Final answers will depend on studies of still more astronauts. [...]

By many measures, the scientists eventually found, Mr. Kelly changed about as much as astronauts who stayed on the space station for only six months. Eventually the pace of biological change slowed, suggesting that perhaps the human body reaches a new equilibrium in space.

But Mr. Kelly’s body was also altered in some surprising ways.

Dr. Bailey studied special sections of his DNA called telomeres, which sit at the end of chromosomes, protecting them from deterioration.

As people age, their telomeres tend to get shorter. Stress — such as radiation or pollution — may hasten aging by fraying telomeres even faster than usual.

Strangely, the average length of Mr. Kelly’s telomeres increased in space, rather than decreasing, as if his cells were becoming more youthful.

Regular exercise and a healthy diet might be part of the reason. But going to space also might have awakened a quiet population of stem cells in Mr. Kelly’s body, Dr. Bailey said. [...]

The strange lengthening of Mr. Kelly’s telomeres disappeared after less than 48 hours on Earth. In fact, Dr. Bailey and her colleagues started finding many cells with telomeres that were shorter than before Mr. Kelly went to space. [...]

These two lingering changes — in Mr. Kelly’s cognition and DNA — left a number of experts concerned about the risks of a trip to Mars, which could take a year.

Obviously, there's much that we do not know.

A full technical report of the study: The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight.