“NASA conducts many detailed studies on the health of their astronauts,” said Nishanth Surianarayanan, a Ph.D. student in mechanical engineering. “But this issue is a relatively recent discovery.”

The issue involves the glymphatic system, a biological process which was first confirmed in living humans in October 2024. Cerebrospinal fluid, the material that comes out during a spinal tap, typically circulates through the brain and spinal cord. It flows around the brain’s blood vessels in annular gaps known as perivascular space, and serves a similar function to blood — bringing vital materials in, and removing metabolic waste products.

“But during long periods of microgravity, that outflow is reduced,” Surianarayanan said. “The fluid flows into the brain, but it doesn’t flow out to remove waste products. As a result, those waste products linger.”