Gravity, Part. 2

It’s all about the bones.

Your bones are a central part of your physiology. They hold your body up. They also help you to move, by giving your muscles something to push against. They also house the marrow that amkes your red blood cells. And, like all systems in the human body, evolution has optimised them for 1 g of gravity. How are they going to react to a hypothetical colony planet, with g=0.68? or 1.3? or 0.17 (if we’re talking about a Moon colony)?

I’ll be clear about one thing; at the moment, we don’t know for an absolute fact. But there is some evidence to suggest that there could be tears before bedtime…


In current space missions, bone atrophy is a major problem for astronauts.

It’s been known for some time that on long space missions, astronauts tend to lose a lot of bone mass. The drops can be quite drastic; one source I’ve seen puts the numbers as high as 15-20% over a six-month mission; another said an average of 1.5% per month. Still, whichever estimate is correct, it’s clear that this isn’t good news. Thinner bones are weaker, more easily breakable bones, and this is Not Good. This is bad enough on short-term space missions, but for a permanent colony on a low-g planet, this could be a major public health issue.

So, we know bones atrophy. But why does this happen?

Well, it’s down to the fact that the human body is a conservative system. It doesn’t like wasting valuable metabolic energy on things it doesn’t need. Basically your muscles and your skeleton are about as strong as they ‘need’ to be. If you increase the load on the system, they will strengthen too. If you decrease the load, they will reduce. How do they do this? The mechanism is via specialist bodies called osteocytes.

Whenever we do anything, it strains the bones. Strains and stresses can produce small amounts of damage. Osteocytes send a sort of ‘repair team’ to these places. Osteoclasts remove the damaged cells and osteoblasts replace them with new, fresh cells. But – and here’s the clever bit – they slightly over-repair each bit of damage. So the net effect is that stressing your bones gradually makes them stronger.

Problem is, an astronaut in 0g doesn’t weigh anything. Stress/strain on the bones is vastly less. So the osteocytes basically shut up shop and go to sleep. And the bones start gently wasting away.

The obvious answer, of course, would be waking the osteocytes up again. Problem is, we don’t know how to do that.

Furthermore, bone loss has implications for other physical systems. For instance, the volume of calcium going into an astronaut’s urine can cause kidney problems. There’s no really good place to get kidney stones, but Low Earth Orbit is probably a uniquely bad one! (Imagine what the multiple g’s during ballistic re-entry is going to do to those … Ouch!)

And no, calcium supplements don’t appear to help much. The body just pees out the excess chalk. It doesn’t ‘think’ that it needs it – even though the mind sat inside the body very likely knows this! Basically what we’re seeing is what happens when you transport an organism outside of the environment for which it is adapted – it starts to misfire in all sorts of weird and unexpected ways.

To be fair, bone loss is probably a surprisingly-mild misfire. I mean, when you get down to it, there’s no obvious reason why zero gravity should be survivable *at all*. After all, nothing in our evolution will have existed to prepare us for it. (The only ‘freefall’ a pre-human ape would ever encounter would be the sort that ends fractions of a second later on the ground.)

That said, bone loss is bad. Should the load ever suddenly increase for some reason, your weakened skeleton will be far less able to deal with it. (Consider the plight of some osteoporosis suffers, who can break bones by shifting their weight from one leg to the other.) This is not good!

But there’s more.

In zero-g, your spine experiences no load. So it decompresses. Astronauts grow by as much as seven centimetres, just through spinal decompression. And this is potentially really bad news. You see, the spine is supposed to be under pressure – taking the pressure off of it is not necessarily good for it. This is why doctors have stopped prescribing bed rest for back pain, incidentally – it’s been shown to make it worse, not better!

So, what happens to colonists on a low-g planet? Presumably, some degree of bone loss is inevitable. If they’re lucky, X-ty years in the future, there’ll be a drug to control this. (But drugs have side-effects. And also, if you forget to take it – or won’t for religious reasons, say – then it can’t help you at all.) Presumably, under not-0g, bone loss stops somewhere – but where? We don’t know. As far as I know, there is no research on this! (We don’t even know if it stops in 0g, either – research on paraplegics on Earth seems to imply that it might, but that isn’t absolutely clear either.)

So, let’s look at this from a low-g colony, public-health point of view. We have a population on said colony. Pretty much everyone has bad backpain. Pretty much everyone has some degree of osteoporosis. On the plus side, the lower gravity might offset this a bit as well – but if it only takes one bad fall to snap your weakened neck, what then?

Now, that’s the low-g case. What about the high-g case?

Well, mice have been raised in centrifuges at 2g. The good news is that the mice could and did live. In fact, raising mice in high-g creates a sort of ‘super-mouse’; it seems the osteocyte problem runs in reverse at high gravity. And of course more weight will bulk up your muscles. So, oddly enough, within some limits higher gravity may actually be better than your skeleton and musculature than low. (Although this won’t necessarily make you feel stronger; remember, everything’s heavier – and you’re carrying around more bodymass than you would be on Earth.)

Of course, the caveat here is that humans aren’t mice, and what applies to mice does not necessarily apply to us. Mice are a lot less massive and so effectively ‘feel’ less gravity than we do. I strongly suspect that 2g is too high for long-term human inhabitation! It might just be tolerable for skinny types like me, but not for everyone!

In addition, there’s another problem with high-g planets: accidents.

Here on Earth, it’s surprising how short a fall that can (in principle) kill you. Falling wrongly off of a bar stool can do it. Or consider the person who runs down the stairs at the station, anxious to catch their train – and takes a tumble halfway down. Now, imagine what both of these would be like on a (say) 1.5g colony. Somehow, I have a suspicion that person isn’t going to end up in hospital unless they’re very lucky indeed!

(Presumably this means running at train stations will be illegal on high-g planets?)

Also, consider the random things that you might accidentally drop on Earth. A beer bottle, a paperweight, a glass, whatever. If they hit your toe, it’ll certainly hurt. On a high-g planet, they’ll be travelling much faster by the time they reach the ground. And kinetic energy rises with the square of the velocity – so you’ll probably be looking at a broken toe, not a bruise!

Now, consider what happens if something falls on your head. Not crack, more crunch, I suspect. One can also imagine stairways on a high-g planet, all of which by law have to have a right-angle bend in them. The purpose of the bend? In case you take a tumble. They’re to break your fall at the point where you could still survive it!

So, on low-g planets we have backpain and osteoporosis. On high-g planets we have exhaustion and deceptively-dangerous accidents. Are these the only things we can imagine going wrong with the human body in different gravities?

Hell no!

Next installment: body fluids and gravity. Or, why different gravities may cause problems in the peeing department…

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