Why Astronauts Stumbled on the Moon

NASA Video: Apollo 17 Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, while Command Module Pilot Ronald Evans orbited above. Launched Dec 7 1972.

This video is one of many you can enjoy in NASA’s extensive multimedia archive. All content is public domain, but should be credited. Your tax dollars at work.

One of the curious things I’ve noticed watching videos of Apollo astronauts on the Moon is that they seem rather clumsy for people with “the right stuff.” Several times an astronaut stumbles and falls, as seen in this video. Sure, the bulky suits and backpacks don’t help, but there’s enough gravity on the Moon to keep them stable — or so I thought until I read this article on Space.com:


Gravity on the Moon is about 1/6 that of Earth. That means things weigh less and fall to the ground more slowly. An astronaut with a full suit would weigh close to 360 lb on Earth, but only 60 lb on the Moon. When they stumble and fall, they drop to the ground 1/6 as fast as they would on Earth, so they have more time to react. No problems there. So what’s with all the clumsy motion and falling? There are two reasons …

Space.com explained how the vestibular system in our inner ear reacts to gravity weaker than that in which it evolved. Turns out that when gravity is reduced to 0.15g (where “g” is normal Earth gravity) the vestibular system cannot reliably tell which direction is “down.” Gravity on the Moon is 0.165g, only slightly above that minimum threshold. That creates problems for some astronauts when they try to move around.

NASA ran a centrifuge experiment with the subjects lying flat on a horizontal spinning disc. At various intervals they were asked to indicate the direction of “down” in the plane of the centrifuge. The correct answer, of course, is the point on the circumference directly under their feet. At “horizontal gravity” below 0.15g the subjects were not able to accurately point to “down” as shown in this graphic:

This effect is further complicated by the mechanics of the vestibular system, which works by sensing the flow of its internal fluids. That flow is slowed by a factor of 6 in lunar gravity, and that delays the time you think are off balance from the time you really are off balance.

The other reason for astronauts’ clumsy movement on the Moon relates to basic physics. Since they weigh 1/6 less, there is 1/6 the friction (grip) between their feet and the lunar surface. But they still have the same inertia in their mass, and inertia resists changes in motion. So in lower gravity, attempts to control body motion using friction between their feet and the lunar surface will be less effective. For an in depth explanation of this effect see my post about basketball on the Moon:


The Apollo astronauts had only a few hours to adjust to lunar gravity. Given the borderline g requirements for the vestibular system, I suspect lunar colonists will eventually adapt to the lower gravity. If and when they return to Earth, will this adaptation improve their sense of balance? If so, will future Olympic gymnasts train on the Moon to compete on Earth? Gymnastics on the Moon could be interesting more interesting that basketball.

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