Astronauts Still Think Gravity Exists in Space
Astronauts over-grip objects in space as their brains retain expectations of Earth’s gravity.
Millions of years of evolution aren't easily erased by a rocket launch.
Recent research conducted on the International Space Station (ISS) found that astronauts consistently over-grip objects because their brains still anticipate Earth’s gravity.
Following the success of Artemis II, these findings are essential for ensuring crew safety during complex manual tasks on future lunar missions.
The weight of Earth’s habits
Humanity has spent its entire history under the constant pull of Earth’s gravity. From the moment our ancestors picked up a stone, our brains have learned to perfectly sync our grip with the weight of an object to prevent it from slipping.
“Through evolution since the invention of tools in human history, and dozens of years of daily practice in human life, the sensorimotor system has been optimized for manipulating weights,” said the authors of the study.
This optimization is so precise that we rarely think about the complex calculations our central nervous system performs every time we pick up our morning coffee mug.
While researchers have studied how humans adapt to weightlessness during brief parabolic flights, there is less known about how the brain handles long-term stays in orbit. As space agencies are preparing for future lunar exploration, understanding manual dexterity is important. Astronauts on these missions will need to handle delicate equipment and perform complex repairs while transitioning between different gravitational environments. If their brains cannot decipher the correct way to handle an object, they risk damaging hardware or failing tasks.
Researchers behind the new study investigated how the central nervous system integrates the brain's deep-seated expectations of gravity into motor commands during extended stays on the ISS. By tracking how astronauts handle objects over several months, the team aimed to determine whether the brain ever truly forgets the rules of Earth, or if it remains tethered to its terrestrial training.
Testing grip force in space
The team followed 11 astronauts during their missions on the ISS. The crew performed an oscillation task using a specialized tool called a manipulandum. They moved objects with masses ranging from 400–800 grams in rhythmic patterns at various frequencies.
On Earth, an individual's grip would increase as the object gets heavier or moves faster to counter the pull of gravity; however, in the weightlessness of space, that downward pull vanishes.
The results revealed an Anti-Bayesian phenomenon; instead of adapting to the zero-gravity environment, the astronauts’ brains interpreted the missing downward pull of Earth as a negative weight, acting as if every object had an inherent buoyancy that might cause it to drift away.
This led the astronauts to overcompensate by squeezing objects much harder than necessary, particularly when moving them.
Since the brain is used to the downward pull of gravity, the total absence of it can feel like an imaginary upward lift. This is similar to the phenomenon that can occur when walking onto a stopped escalator; the brain still expects movement, so the stillness feels like a physical tug in the opposite direction.
“The imprint of gravity remains visible in the way we manipulate objects even after months of living in weightlessness,” explained the authors.
The researchers found that grip force wasn’t just about the weight of the object; it was also linked to the object’s kinetic energy and the potential consequences of a slip. Even in space, a sliding object carries momentum that can lead to accidents.
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When the astronauts finally returned to Earth, they faced a maladaptation period. Their brains struggled to switch back to Earth’s rules, resulting in incorrect load force predictions and a temporary loss of coordination between their grip and the object’s movement.
Future space mission safety
The cognitive imprint of Earth’s gravity appears to lie incredibly deep. The brain does not overwrite its terrestrial programming; instead, it layers new, slightly distorted rules on top of the old ones.
These findings have direct consequences for mission safety, particularly for programs such as future Artemis missions. When astronauts transition from the weightlessness of space to the partial gravity of the Moon or back to Earth, they will likely experience recalibration periods. During these windows, their manual dexterity will be compromised, making them more prone to errors when handling tools or operating spacecraft controls. Targeted training or specific warm-up protocols may be necessary to help the brain switch between gravitational models more efficiently.
While this study provides a rare look at long-term neural adaptation, it is limited by its small sample size and more data is needed to see if these patterns hold across a broader population.
The team plans to investigate how astronauts handle point-to-point movements and how the brain adjusts for skin friction. They also want to understand how humans manage object collisions in microgravity.
Understanding these fine details of manual control will be essential for ensuring that the next generation of explorers can work safely and effectively on the lunar surface and beyond.
Reference: Opsomer L, Vandergooten S, Thonnard JL, McIntyre J, Lefèvre P. Effect of risks, consequences, and gravitational priors on sensorimotor coordination: insights from weightlessness. JNeurosci. 2026. doi: 10.1523/JNEUROSCI.2036-25.2026
This article is a rework of a press release issued by the Society for Neuroscience. Material has been edited for length and content.
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