Gravity, Pt. 3

I was going to be writing a post about the implications of differential gravity for the body’s, ah, excretory systems next. (There are plenty, as it happens.) But then it occurred to me that there was a much more basic and much more obvious problem that should be considered first. It’s called actually being able to move in the first place!


Walking is something we generally take for granted. And yet, upright bipedal locomotion is largely the privilege of human beings. Certainly we’re a lot better at it than almost anything else, and it also has the double advantage of freeing the upper limb pair for tool use.

However, like everything else in the body, walking is optimised for a constant 1g. And as we’ve discussed before, it’s highly unlikely that any hypothetical colony planet is going to supply exactly 1g at its surface.

Walking is the product of several complicated anatomical compromises. The arches underneath your feet are part of it. So are your toes. (You use them a lot more in keeping your balance than you’ve probably ever noticed – people who have toe amputations often have to learn to walk all over again.) There are also other complicated factors like the hip joint, and the arc of swing on the leg. In physics terms, the act of walking is actually a sort of interrupted fall – your body’s forward momentum as you start to tip over helps with the swing of the legs. And under different gravities, things will fall at different speeds – potentially confusing that set-up.

But also, a differing gravity is going to affect your sense of balance.

The vestibular system, within the inner ear, is the organ that ‘senses’ orientation. Within the vestibular system, the ‘semicircular canals’ are sensitive to rotational motion and the ‘otoliths’ are sensitive to linear accelerations. The otoliths are little solid particles on the ends of fine hairs. When you move, they lag behind due to their inertia – and this drags on the hairs, stimulating the ‘kinocilium’ nerve. This in turn relays the appropriate signals to your brain. As for the semicircular canals, they are filled with a fluid which swirls as you move, and this stimulates the hairs that line them. In addition, the brain uses input from your vision as well to create a model of your motion through the environment. This model allows the body to balance as it moves (and it does all this without you ever noticing!).

And on Earth, this all works quite well. But not so in space.

On Earth, the ratio between an object’s weight (due to gravity) and its inertia (resistance to acceleration, one of the defining properties of matter) are constant. Now, inertia will be the same anywhere in the universe – it’s just a property that matter has. But weight, as we’re discussing in these articles, won’t be. This also means the ratio between the two will be off – and this will confuse your sense of balance.

What happens when there’s a conflict between your visual input and your inner ear? The answer is motion sickness – nausea. Why nausea? Because in nature, the only times that your eyes and your ears would contradict each other were if you had ingested a poisonous substance and were hallucinating. At this point, you need to puke, pronto! So that’s what the body does.

Unfortunately, thanks to the wonders of modern technology, we regularly encounter situations today where the ear and eye are brought into conflict. Consider a car going round and round a roundabout, at speed. The fluids in the semicircular canals will be sloshing like no-one’s business, but the eyes insist that you’re remaining on a level plane. Talk about a sensory conflict! It’s no wonder it can trigger the puke-response.

Problem is, on a different-gravity planet, the eye and ear will be in conflict all the time. The weight of the hairs inside your inner ear will be different to their factory settings, so they will be confused from their normal behaviour. But there’s worse. Your inertia will be exactly the same as it was on Earth – so for any motion, for the same acceleration you’ll still have the same stopping-distance. You might feel nice and light on a low-g planet as you bound along in a straight line – but consider what happens if you have to stop suddenly! (This is not a theoretical issue only; it was a problem that was experienced by the Apollo astronauts on the Moon, where inertia=1 but g=0.17.)

All of this will cause trouble for balance and for walking. In fact, I think it’s a reasonable guess that people will regularly fall over.

So, let’s go back to our low- and high-g planet examples from before. We have a low-g colony where osteoporosis is common – and people fall over a lot. We have a high-g planet where broken bones, bruises and painful falls are common – and people fall over a lot. Presumably in both cases people are going to have to learn to move carefully!

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