Our bodies change in ways that cause a lengthy list of health risks for space travelers when we are thrown into freefall for months at a time.
The most recent study of the effects of microgravity on our biology focuses on the areas around the blood arteries that thread through our brain, indicating worrying alterations that astronauts carry with them between trips.
Researchers from throughout the United States analyzed a series of magnetic resonance imaging (MRI) scans of 15 astronaut brains obtained before and after their six-month stay on the International Space Station.
The team discovered that time spent in orbit had a significant impact on the brain's plumbing by using algorithms to carefully assess the sizes of perivascular spaces (gaps in brain tissue thought to facilitate fluid balance). At least for the first-timers.
In the two scans done before the flight and the four scans taken thereafter, there appeared to be no change in the diameters of perivascular spaces among the senior astronauts.
"Experienced astronauts may have established some form of equilibrium," says neurologist Juan Piantino of Oregon Health & Science University.
Given what we already know about how the brain bends when gravity's steady grip is removed, the findings may not be all that unexpected.
Previous research has indicated that brain structures and fluid volumes take a long time to recover after a trip to space, with some modifications lasting up to a year.
At the moment, astronauts seldom go into space more than a few times throughout their lives, usually staying for six months at a time. This might all change when the space sector becomes more commercialized.
It will be useful to know if repeated journeys exacerbate the harm or if the changes astronauts encounter on the initial voyage help them adjust to a new normal.
"We all learned to exploit gravity to our advantage," Piantino explains.
"Our brains were not placed in our feet by nature; they were placed far above. What happens to human physiology when gravity is removed from the equation?"
Even in the case of increased perivascular spaces, it's unclear if the alteration poses any significant health consequences.
When we sleep, we tend to use this neurological drainage system the most. The flushing of fluids surrounding our grey matter appears to be crucial in eliminating waste products that build up during our more busy hours.
Disruptive chemicals may build if these channels do not work properly, thus increasing the risk of neurodegenerative illnesses like Alzheimer's.
It's too early to say whether microgravity has any effect on the circulation of cerebral spinal fluid around our heads, let alone whether changes in the geometry of the network of channels are substantial. It might not even be apparent until researchers have a large enough sample of seasoned astronauts with a long career behind them.
Knowing more about these little alterations extends beyond the risks of operating in the space business off-world.
"It also pushes you to consider certain basic scientific concerns, such as how life began on Earth," Piantino explains.
The constant pull of gravity isn't only something we have to contend with. It's a force we've evolved to use, helping in blood flow and waste shedding, as well as a range of additional tasks we've hardly explored.
We're probably guaranteed to discover more about illnesses and disorders our bodies have been forced to weather down here if we examine the tiny changes in health and morphology under situations we never evolved to bear.
