The National Aeronautics and Space Administration (NASA) said on the 8th that four astronauts staying on the International Space Station (ISS) will return to Earth ahead of schedule due to a health issue affecting one crew member. Since the ISS began continuous operation in 2000, this is the first time that an "early return" has been formalized for medical reasons. The strain that the space environment places on the human body is drawing renewed attention.
It is well known that time spent in space affects the human body. Typical examples include weakened bones and muscles and fluids shifting toward the head.
A University of Florida research team said on the 13th in the Proceedings of the National Academy of Sciences (PNAS) that the position of the brain inside the skull shifts and tilts after spaceflight.
With more plans that extend time in space, such as lunar exploration and crewed missions to Mars, it is becoming increasingly important to precisely determine how the space environment affects the human body, especially the brain and nervous system.
The researchers analyzed changes in the position and shape of the brain by taking magnetic resonance imaging (MRI) scans before and after spaceflight of 15 astronauts who had flown in space. They also compared existing MRI data from 11 other astronauts and data from 24 participants in a "bed rest" experiment that simulates microgravity by tilting the head downward.
The analysis found that after spaceflight or bed rest, the entire brain moved backward and upward within the skull and tilted overall in the direction of being lifted upward. In other words, the brain's center shifted from the face toward the back of the head and toward the crown.
The team assessed the degree of movement across 130 brain regions along coordinate axes and said 107 regions showed marked changes in the front-to-back direction and 88 regions showed marked changes in the vertical direction.
However, the patterns of change differed between the astronaut group and the bed rest group. Astronauts' brains showed relatively greater upward movement, while bed rest participants showed relatively more pronounced backward displacement.
This suggests that the bed rest–based microgravity simulations most widely used on Earth may not fully reproduce the entire range of changes actually experienced in space. The researchers also said, "Some of the brain shape changes observed in astronauts did not appear in the bed rest group."
They also suggested that the observed changes may not be limited to differences seen on imaging. That is because the greater the movement in brain areas related to sensing body position and motion, the greater the tendency for post-spaceflight balance loss.
It is known that when people return to Earth after living in space, the abrupt change in the gravity environment can cause dizziness, postural instability, and balance problems. This study offers a clue that those symptoms may coincide with subtle shifts in brain position.
The researchers said, "This study provides data that help more precisely understand the impact of microgravity on brain anatomy," adding, "Further research is needed to prepare for the era of long-term space exploration, including changes by flight duration and frequency, and whether training and countermeasures to reduce balance decline are actually effective."
References
PNAS (2026), DOI: https://doi.org/10.1073/pnas.2505682122