Prolonged exposure to low gravity alters the structure of the brain, the long-term effects of these changes are unknown.
The peer-reviewed paper published in the journal Scientific Reports shows that extended time spent outside of Earth's gravity causes changes in the brain's white and gray matter as well as its free water distribution.
Astronauts who spent two weeks in space saw the least change, while after six months changes began to taper off. These changes take the form of expanding ventricles, ventricles carry important fluid to and from the brain allowing it to properly function.
Although the size and shape of the ventricles are controlled by the body in a normal Earth environment, in low gravity the fluids tend to shift upwards causing the brain to rise higher in the skull and expanding the ventricles.
Ventricles also expand with aging so their short-term expansion should not be of too much concern, however during space flight they expand at a rate far exceeding that of the aging process.
The researchers found that it can take up to three years for ventricles to return to their normal size once back on Earth. The research also suggested that if this was not observed there was less of an increase during space missions, meaning they had not properly reset since the last time. It is still unclear exactly what long-term effects this will have on astronauts.
So what does that mean for astronauts?
Researchers were hesitant to discuss the long-term effects of expanded ventricles as they had not been studied in depth.
Practically this means that they recommend a three-year interval between space flights in order to avoid as of yet unknown long-term effects.
Their research suggests that any changes caused to the ventricles by low gravity can be reversed through a return to normal Earth gravity.
They recommend that astronauts take three-year intervals between missions in order to avoid possible side effects, this will naturally have consequences for space travel and mission schedules.
It will also have an impact on the nascent space tourism industry, if prospective space tourists will have to wait three years between trips it will inevitably limit the market for such trips.