When watching NASA’s historic video footage of the Apollo astronauts on the moon, it becomes clear that the alien lunar gravity was a little troublesome. The moonwalkers would often trip, stumble and wipe out as they explored the dusty landscape.

Now, in a new study published in the journal PLOS ONE today (Sept. 3), researchers suggest that it wasn’t just the NASA astronauts’ unfamiliarity with the strange lunar environment that knocked them off their feet — it may have been their brains lacking a gravitational reference for which way was “up,” possibly causing a loss in perception.

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“The perception of the relative orientation of oneself and the world is important not only to balance, but also for many other aspects of perception including recognizing faces and objects and predicting how objects are going to behave when dropped or thrown,” said Laurence Harris of York University, Toronto. “Misinterpreting which way is up can lead to perceptual errors and threaten balance if a person uses an incorrect reference point to stabilize themselves.”

Through a series of European Space Agency centrifuge experiments funded by the Canadian Space Agency, Harris and co-investigator Michael Jenkin (also from York University) were able to simulate gravitational fields of different magnitudes. Subjects being accelerated by the centrifuge were then asked which direction they thought “up” was. Through this perceptual test, Harris and Jenkin discovered that “the threshold level of gravity needed to just influence a person’s orientation judgment was about 15 percent of the level found on Earth,” according to a university press release.

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Interestingly, 15 percent Earth gravity (or 0.15 G) is very close to the gravity experienced by the Apollo astronauts on the lunar surface — 0.1654 G or 16.5 percent of Earth’s gravity.

Although this is likely only one of the many reasons why astronauts on the moon often spent time eating moon dirt — other reasons possibly include lack of maneuverability in bulky spacesuits and restricted visibility while navigating the rocky terrain — understanding how the human brain responds to different gravitational fields is essential if we are to become a multiplanetary species.

“If the brain does not sense enough gravity to determine which way is up, astronauts may get disoriented, which can lead to errors like flipping switches the wrong way or moving the wrong way in an emergency,” said Jenkin. “Therefore, it’s crucial to understand how the direction of up is established and to establish the relative contribution of gravity to this direction before journeying to environments with gravity levels different to that of Earth.”

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Fortunately for future Mars explorers, say the researchers, their balance should be just fine. The Martian gravity of 38 percent that of Earth’s is more than sufficient to aid future Mars colonists’ perception of orientation.

For one famous example of falling over in lunar gravity, watch how Apollo 16 commander John Young expertly dealt with his stumbles as he probed the surface layers of lunar regolith: