Through an analysis of blood samples taken from five cosmonauts before and after extended stays on the International Space Station (ISS) – each spending an average of 169 days in orbit – researchers observed elevated concentrations of three biomarkers linked to brain damage after the return to Earth.

"This is the first time that concrete proof of brain-cell damage has been documented in blood tests following space flights," says neuroscientist Henrik Zetterberg from the University of Gothenburg in Sweden.

More data from more people will be required to figure out exactly how and why time in space affects the brain negatively.

This small study doesn't go into why these changes in the brain might be happening – with just a handful of people in space at any one time, it continues to be difficult for researchers to gather a broad enough set of results to make any detailed conclusions about the health risks of space travel.