[jerf]

Artificial gravity is easy. You "just" spin things.

The tricky bit is being able to launch enough mass to build something safe and comfortable to spin; there's a limit to how small you can make your spinning habitat before the difference between "centrifugal force" and gravity is too pronounced for our long term comfort. That's part of why making it cheaper to launch per unit mass is so important. If we could put ten times the mass in space for the same price, the ISS would probably look quite different.

I'm of the same opinion as you in general; what spending years in zero-g has proved is that it's not long-term viable. There's too many ways in which it is not viable to expect us to be able to fix all of them, when indeed it's not clear we can fix any of them with drugs or anything short of massive genetic engineering. We don't know how much gravity is necessary, though I'm inclined to guess closer to .5G than .05G. Once you get enough mass in space, though, that's not really that difficult.

[jfoutz]

Two skylabs and a really long rope seems pretty achievable. The full wheel from 60's sci fi looks very cool, but two weights on opposite sides of a string seems so much simpler.

[lttlrck]

This idea is used in Seveneves

https://en.m.wikipedia.org/wiki/Seveneves

[ams6110]

Now all your space vehicles need to be able to hold together under 1G of force. That means a lot more structure and mass and the cost goes way up.

[jerf]

I didn't spec 1G of force. Though, as I, ahem, already said, I do suspect we'll need closer to .5G than .05G. But while it's not quite as evil as the rocket equation, you do get non-linear advantages as you go down the gravity scale. .5 is already much less than half as hard as 1G, and we could perhaps get away with .25.

Here we have a chicken and egg problem; how can we launch the variable-speed lab we really need to figure out how much gravity we need if we can't afford the 1G lab in the first place? Because proper science suggests we ought to be able to test the full range up to 1G. I'm spitballing .5 or .25, but scientifically speaking there's no guarantee the optimal won't be .8, 1.0, or, conceivably, even 1.1 or 1.2G. (Sure, the latter is unlikely, but I can't scientifically rule it out a priori.)

[JoeAltmaier]

How about sleeping in a centrifuge? No need for the entire spaceship to spin.

[nxzero]

If it was possible to build a gravity sleep chamber, it's very possible that exposing the human body to gravity and zero-gravity on a daily basis would be put more, not less stress on a body than zero-gravity alone.

[JoeAltmaier]

That's just a random guess. We already lay down for 8 hours and stand up for 16. My random guess is, our fluids would recover with at least some artificial acceleration and why not when sleeping?

[rbanffy]

If you intend to replicate the effects of standing, you'd need to sleep standing.

[kybernetikos]

Chris Hadfield says that sleeping in 0g is one of the best bits of being in space.

[hoorayimhelping]

>The tricky bit is being able to launch enough mass to build something safe and comfortable to spin;

Is it? I might have a completely busted mental model, but I thought you only need a module-sized mass on one end of a rod the length of things we've already assembled in space (e.g. an ISS truss) and a motor to spin it.

[jerf]

It gets complicated, and the mass budget starts going up fast, if you want other things to be able to dock to it. Two space stations on the opposite side of a long rope (per the other reply) is also not a very compelling story to tell to Congress when you have to admit that astronauts won't be able to travel between those two space stations to speak of, unless, again, you really up the mass budget.

We're space-poor. It's just too darned expensive. Even relatively simple designs are out of our reach right now, if you have to manifest them in real designs with real safety margins and real practical applications, such as being dockable.

[nxzero]

What about grabbing mass from the surface of the moon, junk that's floating around, etc.?

Seems like this might be cheaper than getting it off Earth, right?

[akiselev]

Except then you'd need to launch a full blown transport, mining, and manufacturing facility from Earth and assemble it just like the space station. I wouldn't be surprised if such a ship is heavier than the station you're trying to build, even if you strip out life support because the gear necessary to make silicon chips or machine hard metals is massive and numerous.

[doikor]

At first we don't want 2 space stations spinning. We want 2 space crafts spinning and traveling to somewhere where we have enough gravity to survive (mars). All humans on one ship and cargo/return counter weight as other ship linked together by a tether and spinning for the travel duration.

But yeah we still need to lower the launch cost per kg to make it really feasible and super heavy launch vehicles which don't exist at all at the moment. Realistically we would want to launch 40+ tons directly to mars from earth (Falcon Heavy should be ~13 tons so we would need around 3x the power of that). We don't have anything with enough delta v to do that at the moment.

[thaumasiotes]

> Two space stations on the opposite side of a long rope (per the other reply) is also not a very compelling story to tell to Congress when you have to admit that astronauts won't be able to travel between those two space stations to speak of

I don't get the complaint. Assuming travel from one station to the other is impossible, what's supposed to be wrong with having two smaller stations that don't cripple the health of the inhabitants instead of one bigger one that does?

[jerf]

Congress won't fund two stations.

Again, the problem isn't physics or engineering, it's that we're poor in space. This, and a lot of the other posts, are basically saying "What's so hard about having a job 10 miles away? Just drive there!" to people too poor to own a car, too poor to even dream of owning a car. Yes, it is a simple problem... if space wasn't so expensive to us. (I mean, not trivial, we'd still have to redesign a lot of stuff, but there's no reason to believe there's a fundamental problem.)

[thaumasiotes]

This will serve as a reply to you and wlievens:

The objection I questioned was "when you have to admit that astronauts won't be able to travel between those two space stations to speak of". You and wlievens are both pointing out that the scheme is unworkable regardless of the necessity of traveling between one station and the other. That's fine, but it doesn't respond to my question of "who cares that you can't travel between the stations?" It means I was correct to wonder how it could be relevant that travel between one station and the other is impossible. The objection I questioned makes no sense.

On a separate note, if Congress will fund one station, they can't object to a two-station system at the same cost. The number of stations is, again, not relevant to much.

[wlievens]

Still can't dock. You need to be able to dock in the stationar center module.

[thaumasiotes]

> there's a limit to how small you can make your spinning habitat before the difference between "centrifugal force" and gravity is too pronounced for our long term comfort

There is no such difference. The constraint you probably have in mind is that you want the force of gravity at your head to be the same as the force of gravity at your feet. This is a problem with actual gravity too; see https://en.wikipedia.org/wiki/Spaghettification .

[jerf]

No, the vestibular system gets "annoyed", which is to say, permanently motion sick, if you spin something too small. Our vestibular system is not designed to deal with Coriolis force. You have to keep it below a certain threshold or you'd rather not spin at all... which is, in some sense, exactly why we don't spin the ISS, or, rather, designed something ISS-sized to spin. It's too small. You need a certain size in the rotation axis.

If the solution was just to spin our tin cans the problem would be solved.

[thaumasiotes]

From what I read on Wikipedia:

- The Coriolis force is a different effect than the centrifugal force.

- The Coriolis force is unrelated to the radius of the rotating object. This is not true of centrifugal force, so while it doesn't make sense to talk about Coriolis forces resulting from spinning "something too small", it does make sense to talk about them from spinning "something too small" subject to the constraint that the apparent gravity from the centrifugal force meets some threshold such as g.

- The Coriolis force is a tidal effect, inasmuch as it is described by the tidal equations of Laplace ( https://en.wikipedia.org/wiki/Theory_of_tides#Laplace.27s_ti... ). That would make it an example of the tidal constraint that I originally suggested.

Have I made a mistake somewhere? The third point seems kind of shaky.