Why do astronauts fall so easily on the moon?

It appears that when astronauts walked on the moon, they were very prone to losing their balance and falling over and finding it hard to get up.

So why is this? It turns out that being able to accurately judge which way is up is crucial for maintaining balance and via Kiki Sanford, I found this paper that examines the relative contribution of the various factors that lead us to judge whether we are upright.

According to the paper’s authors:

Establishing an “up” direction is a multisensory process that integrates information about orientation obtained from visual cues, gravity and the internal representation of the body [16]. Gravity typically contributes about 20% to the perceptual upright (PU: the direction in which polarized objects, including such things as writing, trees and people, are judged as being the correct way up) with the remainder coming from visual cues and the orientation of the body [17]. Many studies have estimated the threshold for detecting linear acceleration [18]. Estimates of this threshold vary considerably depending on the methods employed [19] but there is a general agreement that accelerations along the long axis of the body above about 0.15 m.s−2 (0.02 g) are reliably detectable.

To assess how much gravity is needed to establish an up direction, we had participants view a highly polarized visual scene while lying supine on a human centrifuge (Fig. 1a). We rotated the centrifuge at various speeds to create controlled, maintained linear accelerations along the long axis of the body (Fig. 1b). The visual scene they were viewing could be rotated about the naso-occipital axis, which had the effect of pulling the perceptual upright away from the body’s axis towards the direction indicated by the visual background. As artificial gravity is added along the body’s axis, there is a corresponding reduction in the relative influence of vision

We find that, even when the simulated gravitational force was above the acceleration threshold, it was only effective at influencing the perceptual upright above about 0.15 g: indeed, the gravitational force on the moon would only barely be able to provide adequate gravitational cues necessary for orientation.

Since the moon’s gravity is 0.16g, very close to the threshold of 0.15g, this means that astronauts are much more dependent on visual cues to maintain their upright posture. Given the barrenness of the scenery and the limited view allowed by their spacesuits, they could easily lose their sense of which way is up.

On the positive side, they would be able to stay upright far more easily on Mars which has gravity of 0.38g.


  1. DonDueed says

    Another factor in falling is the reduced friction due to the low gravity. Friction is a function of the force between two objects (among other factors), and the force that presses the moon-walkers’ foot against the soil is much lower than on Earth. Furthermore it’s spread out over a larger area (big klunky boots) so the force per square centimeter is lower still.

    That same lower force reduces the sensation of pressure on the astronaut’s foot.

    Add to that the somewhat slippery nature of the top layer of regolith — moon dust — and you have the makings of very poor footing.

  2. busterggi says

    I seem to remember a documentary about an astronaut that fell off the moon all the way back to Earth. Or maybe it was in Little Nemo.

  3. weaver says

    Another factor is the suits they were wearing – their packs were extremely heavy in relation to their bodies, and the center of gravity of their body and suit combination was not the same as their normal center of gravity. Combine this with the low surface gravity and low friction, and it’s rather impressive that they were able to learn to walk so effectively as quickly as they did.

  4. AsqJames says

    As well as restricting their vision, the spacesuits will have restricted their movements to some degree and I’m sure the weight distribution would have significantly altered their centre of mass – what’s the mass of that back-pack?

    So: reduced sensory input, reduced vision, reduced friction underfoot (per DonDueed @1) and a higher & further backward centre of mass/gravity. It’s lucky they were athletically fit and had hundreds of hours of training or they might have spent hardly any time upright at all!

  5. Lassi Hippeläinen says

    #1 – sorry, but your intuition goes wrong.

    Remember getting your driver’s license? When braking, the weight of the car doesn’t matter. It increases the friction, but it also increases the kinetic energy to be killed, so the distance travelled before stopping is the same.

    Surface area of the foot doesn’t have an effect either. There are additional non-linear effects, like how the moondust behaves under stress, where the footprint and its grooves matter.

    I’ll add one more reasons for having difficulties in keeping balance: timing. In the Moon movements should be slower than here. Earthly reflexes are so fast the corrective movements overshoot. That may be why the astronauts learned that it is easier to move around with kangaroo jumps than by walking.

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