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This is a very critical question that is necessary to get answered for flights to distant celestial bodies. The inner ear liquid makes you perceive which way you're pulled onto a celestial body. This makes you keep balance when standing or walking. We know that on the Moon at about 1/6 g the astronauts sometimes lost balance and fell. Its gravity seems to be close to the limit of where you can walk stable. Do we know where the limit is where your inner ear liquid would fail and you'd have to be very careful when walking on a body to not fall?

Is it possible that the limit is around 1/8 g (in that case Callisto would barely be within)?

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    $\begingroup$ You make a claim about 1/8th gravity being the limit, what do you base that on? $\endgroup$ – lijat Mar 22 at 15:13
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    $\begingroup$ If the inner ear liquid would fail in zero gravity, stays at ISS, the International Space Station for many months would not be possible. $\endgroup$ – Uwe Mar 22 at 17:38
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    $\begingroup$ We got a system to feel rotations and another system to feel gravity and accelerations. Which system do you think is failing in zero gravity? It is not the liquid that is failing. $\endgroup$ – Uwe Mar 22 at 19:02
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    $\begingroup$ Welcome to Space! According to the Apollo Program Summary Report, astronauts fell because the spacesuits were cumbersome and the lunar soil was sometimes slippery. It had nothing to with inner ear function. In fact, the Apollo 14 astronauts could sense that their lunar module was leaning by 7 degrees. Motion sickness medications were used only during the weightless portion of the flight, in the command module. $\endgroup$ – DrSheldon Mar 23 at 3:22
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    $\begingroup$ @uhoh: They noticed the tilt while they were trying to sleep, so visual cues didn't apply. It does report that they looked out the windows to reassure themselves that the spacecraft wasn't tipping over. That sounds to me like they were sensing the tilt using their sense of balance, and not other factors. $\endgroup$ – DrSheldon Mar 23 at 3:54
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Your cited article is not a good model for walking in low gravity

The article you cited describes a study using the Short Arm Human Centrifuge at the ESA training center in Colonge, Germany. Here is a picture of that facility from the web page of the contractor that installed it:

SAHC

The red object is one of four couches for human subjects. They recline with their heads downward and away from the center, and are then spun on the 6 meter diameter centrifuge at 40 revolutions per minute.

According to your cited article,

While whirling around in the centrifuge, the participants took a perceptual test, in which they had to choose whether the tilted letter they were looking at on a circular computer screen was a "p" or "d." In a near-zero-gravity environment, such as the International Space Station, astronauts need to rely on visual cues alone to orient themselves, Harris said. The results of the experiment found that gravity only starts influencing a person's sense of up and down once it hits about 0.15g.

The problems I have with this study are:

  • It measures visual perception. There is no attempt to simulate walking.
  • It rotates at 40 RPM. That is completely unlike walking on a moon or asteroid. The vestibule of the inner ear can sense the rotation.
  • Experiments are limited in time to just a few hours. Measuring long-term effects are not feasible.

There's no good way to study walking at that much gravity

This PDF from the NASA Partial Gravity Analogs Workshop lists every known method of creating less than 1 g. (There isn't space here to describe each of these methods; read the paper for that.) Each method has flaws for simulating walking in low gravity:

  • Not enough space for a human subject (short-arm centrifuges on ISS, rotating-wall vessel)
  • Method does not last for long enough time (parabolic flight, free-fall wind turbine chamber)
  • Could not create low enough gravity (short-arm centrifuge on STS-90 was 0.5 g and 1.0 g only)
  • Can only create gravity greater than 1 g (rotating rooms)
  • Method merely unloads weight off the feet, but does nothing to eliminate the pull of Earth's gravity on the otoliths (bed rest, harnesses, overhead suspension, head-out graded water immersion, head-out graded dry immersion, whole-body weighted-garment water immersion, lower body positive pressure, free-fall wind turbine chamber)
  • Tilts the subject so that the pull of Earth's gravity on the otoliths are at an unnatural angle (vertical treadmill, head-up tilt, supine/head-down short-radius centrifugation)
  • The subject rests against a surface, which interferes with attempting to walk (bed rest, head-out graded dry immersion, head-up tilt, supine/head-down short-radius centrifugation)
  • The subject is immersed in water or other viscous medium which can interfere with walking (head-out graded water immersion, whole-body weighted-garment water immersion)
  • The low-gravity effect changes with posture (free-fall wind turbine chamber)
  • Involves rotation, which can be sensed by the vestibular organ (rotating-wall vessel, short-arm centrifuges, long-arm centrifuges, rotating rooms)
  • Cancelled because the vibration loads would exceed ISS limits (Artificial Gravity Research with Ergometric Exercise)
  • Theoretical, not experimental (physical modeling, computational modeling)
  • Concept only, often used in science fiction (long-arm centrifuge in space)

Finally, there is a misconception in the question that Apollo astronauts fell on the moon because of the effects of the 1/6 gravity on the inner ear. Actually, the astronauts fell because the spacesuits were cumbersome and the lunar soil was sometimes slippery.

Most crewmen preferred the loping movement. When the loping movement was used, the inertia of the crewman wearing the extravehicular mobility unit (representing an earth weight of 360 pounds) and the sometimes slippery effect of the lunar regolith required the crewman to plan for a finite stopping distance in advance of the selected point.

[...] The Apollo 11 crew noted that egress operations around the hatch, porch, and ladder were performed easily without losing body balance.

Apollo Program Summary Report

No medicines for motion sickness were taken while on the moon.

Indeed, inner ear function on the moon was good enough that the Apollo 14 astronauts could not sleep because they could sense that their lunar module was leaning 7 degrees.

The tilt was especially noticeable during the sleep periods and made sleep difficult because the crew was uneasy in this awkward position.

Apollo 14 Mission Report

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  • $\begingroup$ I didn't even know STS-90 had an artificial gravity centrifuge on board. It led me the following text: books.google.at/… $\endgroup$ – user35272 Mar 24 at 6:42
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    $\begingroup$ The above text states that Apollo astronauts were indeed confused on what's down and up and that Apollo 11 astronauts didn't notice the 4.5 degree tilt of their LM. It also tells that 0.007g is about the minimum value for humans to exprience linear gravity (that's circa the gravity of Proteus and Nereid) properly while 0.22g is the minimum value for rotational gravity. $\endgroup$ – user35272 Mar 24 at 6:45
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    $\begingroup$ The STS-90 centrifuge was basically a spinning chair. No one was walking around in it. There is a video of it here: spaceflight.nasa.gov/gallery/video/shuttle/sts-90/mpg/… $\endgroup$ – Organic Marble Mar 24 at 13:01
  • $\begingroup$ @OrganicMarble You can't walk in a big centrifuge either. :-) $\endgroup$ – user35272 Mar 25 at 12:24

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