# What Is Microgravity?

I have some thoughts, over at the SpaceTech Analytics blog.

## 31 thoughts on “What Is Microgravity?”

1. MichiCanuck says:

I’m reminded of Larry Niven’s short story “Neutron Star”. What could possibly reach into a General Products hull and kill the crew?

2. David Spain says:

I’m reminded of ST “The Naked Time”. What could large gravity gradients do to ordinary water?

1. MichiCanuck says:

Oddly enough, I have a paper in review that addresses this very issue. π

1. David Spain says:

Pre-print?

1. MichiCanuck says:

Not yet. I don’t want to screw the pooch. The revised version is in review right now and that can be a delicate time. A hint: a high gravity field also produces a high pressure gradient in a body of water.

1. David Spain says:

Dude! Surf’s up! π

3. George Turner says:

Oddly, that site doesn’t show the author’s name, at least as far as I can tell.

Anyway, unless I’m mistaken, free-floating objects on the ISS are at zero-G in between the times they bump into a wall. Floating objects outward of the center of mass will get bumped along by the trailing side, while objects inward of center will keep bonking into a wall on the leading side.

1. Daver says:

Well, if it were a vacuum (or a reasonable facsimile thereof) inside, but then you have bigger problems. I think the acceleration due to air drag is going to be higher than that caused by the gravity gradient.

4. gbaikie says:

“When you are on the surface of the planet, it wants to pull you toward its center. ”

Well also your mass also wants to pull it towards you.
Gravity is a two way street.

1. gbaikie says:

Btw I was hoping it would discuss what interests me at the moment.
Which is artificial gravity is not the same as gravity {or natural gravity].
Now, there is two kinds artificial gravity, the type from spinning and acceleration in one direction. Einstein one could measure the difference between acceleration in one direction and gravity {maybe mostly true] but you tell you are spinning.

Anyhow, I have wondering income housing in the Ocean and lately about low income housing in Space {such as Venus L-1} but that depends on cheap, spinning artificial gravity. Or getting artificial gravity by spinning seems cheaper than the other way to make artificial gravity.
But we don’t actually know if any artificial gravity has same biological effect as real gravity. Nor do we know if 1/3 real gravity works.
And with low income, was thinking using much less the 1 gee of artificial gravity, as less artificial gravity is cheaper- assuming it actually works.

1. cthulhu says:

If youβre accelerating in locally flat space (no other masses about), there is no tidal component to the acceleration.

If youβre under acceleration due to a point gravity source (or an effective point gravity source like being outside a spherical mass) the acceleration vector will always point to the center of the gravity, so objects in free fall will eventually converge.

If youβre under acceleration due to being at the end of a rotating arm, in locally flat space, you can determine that the acceleration drops off as you get closer to the center of rotation, and you can also measure the Coriolis force on a mass thatβs moving.

For space colony artificial gravity, rotation seems like the most practical method, and my suspicion is that a big challenge will be getting them large enough so that Coriolis effects on our vestibular system are negligible.

1. gbaikie says:

–cthulhu
September 8, 2021 At 5:55 PM
If youβre accelerating in locally flat space (no other masses about), there is no tidal component to the acceleration.–

What counts as locally flat space?

And is this “no tidal component” related to idea that one might be able travel thru a black hole if a black hole is large enough?

1. gbaikie says:

Oh, that seems like a complicated math thingy:
https://en.wikipedia.org/wiki/Local_flatness
And:
“This form gives rise to a very precise mathematical description of Einstein’s equivalence principle in GR when Ξ³ is a timelike geodesic.”
https://math.stackexchange.com/questions/1971494/understanding-the-difference-between-a-flat-a-locally-flat-and-an-euclidean-spa

But it does seem related to my question.
But I guess could frame it this way, Earth’s gravity might tie to biological processes which could be upset if it’s a fake Earth gravity.
If believe in God it would seem like a bad idea to do it this way.
Or my understand is you could call the surface of Earth a flat space. But only in relation whatever doing with your math, but obviously Earth surface is not actually a flat space.

2. cthulhu says:

βFlat spaceβ in this context means no significant masses around, so the constant acceleration is the only significant source of acceleration on the test mass. If there are significant masses around, the acceleration field will no longer be uniform due to the other masses curving space time, and given sufficiently accurate instruments, you can distinguish gravity from inertial acceleration.

Another way to think about the equivalence principle is that the test mass is far enough away from any other masses so that the gravitational field is uniform; i.e., the divergence of gradient of the gravitational potential is so close to zero as to be negligible. In that scenario, the equivalence principle says acceleration due to gravity and acceleration due to an applied force are equivalent and indistinguishable.

On the physiological topic, I suspect that living at relatively low g for extended periods of time will cause physiological difficulties. As our host has commented before, this is a research area of importance that is pretty much being neglected.

2. Daver says:

I suspect people would get used to it fairly quickly, like how (most) people get used to living on bobbing boats. The lower the rotation rate, of course, the faster they’d get used to it.

I think it’s still an open question about just how fast of a rotation rate people can learn to tolerate.

1. gbaikie says:

That is what NASA is currently doing.

But the human brain can ignore anything- probably, it’s superpower.
But brain is not really the issue, it’s the rest of human body and long term effects.

2. Daver says:

I can’t reply to gbaikie, so I guess I’ll talk to myself.

I don’t imagine there would be many physiological effects from the head being at .9 g and the feet at 1.1–I suspect that what’s important is that there’s a significant load on the bones. But, of course, I don’t know.

I’m less certain about the head at .19 and the feet at .21–that seems like it could have pretty major effects.

3. gbaikie says:

Ok, got what is meant by “locally flat space” as for:
“and my suspicion is that a big challenge will be getting them large enough so that Coriolis effects on our vestibular system are negligible.”
I don’t think that very difficult but if needed 1 Gee and making longer is a bit of problem. But don’t know if 1 gee and large distance and spinning works. And at moment {because we soon to explore Mars] it would good to know effectiveness of artificial gravity, not at 1 gee, but at Mars gee.
Which I think would very cheap to do. One falcon-9 launch and bunch crew dragon trips to it. And this would better as testbed for Mars exploration program. But we should still explore the lunar polar region, and would be helpful in that regards, also.

5. gbaikie says:

Daver
I googled “microgravity effect microbial life” got:
“By observing the health of astronauts that travel into space, scientists have learned that microgravity has important effects on the human body, causing substantial changes to our bones and muscles. However, scientists have also found that microgravity has dramatic effects on far smaller living organisms, such as bacteria”

But if just about bones and muscles why doesn’t all their exercise solve the problem.

But my whole point is we assume artificial gravity will act the same. I didn’t even question it, I assumed it was the same thing. And it might be close enough to be the same thing.
BUT it seems if NASA send crews to Mars, they should first, check it out.
Actually, I think Musk should check it out, since he wants to live on Mars.
It would cost him, about 100 million dollars.
He could just think of as PR stunt. Do something sciency to indicate how Bezo is some caveman, or whatever. So a cheap PR thing. And it could part of getting the people who going to Mars, ready for Mars.

1. gbaikie says:

Or it’s better than putting a greenhouse on Mars.
And probably one could even make money from doing it.

The other thing is it’s part of solving the 2 year window to Mars.
Put mars artificial station at Venus, and one can have 1 year instead 2 year window, to Mars from Earth and from Mars to Earth.
To do that, you should test Mars artificial gravity in LEO.
Because if time right one get to Mars. Earth to Venus to Mars about fast as Earth directly to Mars. But that rare, typical you have stay in venus orbit a month or so {and taking longer. And going to Venus to Mars would use more rocket fuel.
NASA should do it, to provide abort option back to Earth- as they are keen on that stuff. But Musk did say he wanted to something about 2.1 year window to Mars. Venus could also be market for Mars water.

2. Daver says:

Yep. We know that near weightlessness is a bad thing. We don’t know about 1/6 or 1/3 g. It seems that it would be nice to know before starting a colony off-planet. A Mars base or Moon base gets a lot more expensive if it has to be built in an underground centrifuge.

1. Michael S. Kelly says:

My idea is to build a 1 mile diameter centrifuge here on Earth, and train Olympic athletes in it under a steady 3 g for a couple of years. When they get out, they could leap tall buildings in a single bound…

1. Paul Milenkovic says:

A steady 3 g feels like many workplaces where people are employed?

I don’t think you feed like leap anything when you leave such a place.

2. Daver says:

You trying to raise Jinxians or Des Plaineans on Earth?

6. Jim Davis says:

Rand, terms like zero-g and microgravity are not intended to imply that there is an absence of a gravitational field anymore than the phrase “pulling several gees” implies that that the gravitational field got several times stronger.

All these terms refer to what a standard instrument, a g-meter, reads. In orbit (and in many other circumstances), a g-meter will read very close to zero so terms like zero-g and microgravity are entirely appropriate. It has nothing to do with the strength of whatever gravitational field one is in.

7. I was never aware that microgravity was a subunit of metric gravity, and never heard anyone refer to centigravity in that context either. Kilogravities, megagravities. gigagravities, etc.? Just wondering if its a useful term of art. It’s arbitrary, because G is Earth-centric. Does Planck’s constant figure in gravitational fields somewhere?

1. David Spain says:

πΊ=β^3/πππβπ‘2π

The πΊ
G
constant can be related to Planck’s various constants,

πΊ=β3πππβπ‘2π
G
=
β
P
3
m
P
β
t
P
2

where βπ
β
P
is the Planck length, ππ
m
P
is the Planck mass, and π‘π
t
P
the Planck time.

G = 6.673×10^-8 cm^3/g sec^2

volume per mass acceleration

Its an interesting correlation……

1. David Spain says:

Whoops. That wasn’t supposed to post. iPhone issues. Sorry. Rand delete the above if you can. But since I’ve already stepped in it, I’ll try again.

It’s hard enough trying to write simple text into a WordPress comments box let alone mathematical formulas. I’ll do my best. All appearances of π are subscripts. Expressimg the universal Gravitational Constant in terms of Planck units:

G = β³π / mπ * t²π

where: βπ is the Planck length
where: mπ is the Planck mass
where: π‘π is the Plank time.

G = 6.673Γ10^-8 cm^3/g sec^2

8. gbaikie says:

It seems that artificial gravity concerns a stick rather than a wheel.

I see no advantage in making an artificial gravity structure in the form of wheel.

In terms launching anything from Earth, you don’t launch a wheel, you launch a stick.
But if we lived on the Moon, one would still launch a stick.

A question could be what is the tallest stick which can be launched from the Earth surface?
I don’t we need the tallest stick to be launched from the earth surface. But the Starship will be the tallest stick we have ever launched.
And it seems if NASA were to explore Mars anytime soon, NASA needs the Starship to be launched.
But in terms of artificial gravity station this tallest stick, is going to leaving the largest portion of the stick on the Earth surface, rather taking into orbit.
A stick is the cheapest way to make artificial gravity station.

And the cheapest way to put a long enough stick in orbit, seems to be to use a falcon 9 or falcon heavy.

But what I call a stick is a pipe. And launching tall pipes to the lunar surface, seem like it would important regarding solar power in the lunar polar region.
The milestone of getting lunar tower over say 200 meter tall could significant. Though one do this by putting pipes within pipe.

So I am more interested in doing stuff on the Moon [rather than Mars] if the lunar poles have mineable water. But recently I have been thinking topic of low income housing on the ocean- and that involves “sticks” or pipes. But for much longer time, I was interested in what I call a pipe launcher. Which is about launching rockets AND doing stuff in the ocean. A pipe launcher would be used in the ocean. Though one could dig a deep hole on land and put water in the hole.
Anyhow I was thinking putting pipes within pipes to assemble a stick type artificial gravity station- putting them in the Starship, which has tall space within it. But before Starship I was simply think of extend the height of any second stage rocket [for poles to be used on Moon or whatever]
But I try to be “less extreme” and recently was thinking one make the falcon 9 second stage, a mere 20 meter longer. Which would widest and longer stick ever launched from Earth.

1. gbaikie says:

I thought I could be wrong, as I forgot about Skylab. So looked it
up. It apparently was 25.1 meter long.
Also there was some black programs which might had tall aspect to them. So Skylab was taller than 20 meters. But didn’t include the stage which brought there. So, I was sort of wrong. But you would
keep the second stage as part the stick space station which have total length of about 40 meter tall. And once get crew to it, and take for a few spins. There could option of refueling the falcon 9 second stage, for am extended mission of going to Earth high orbit. And/or Venus or Mars orbit.

1. gbaikie says:

Oh, I was wondering if could send crew with it to Mars or Venus
orbit. But I am thinking now, it’s a bad idea. Or one should probably just send in the simple hohmann transfers {taking a long time getting there, and you want to end up in a high orbit rather than a low orbit around Venus or Mars}.
But the structure I as thinking about only weighs about 10 tons-
So you fill up with cargo stuff. And want to put the most massive part cargo near bottom {or in two lowest floors].
It seems even if radiation and gravity are not problem, you should send crew to Mars as fast as possible.