# Gravity of the situationFebruary 24, 2012 12:41 PM   Subscribe

Would artificial gravity be enough to contain an atmosphere?

If one were able to create an artificial gravity like you see in sci-fi shows like star trek, would that be enough to keep an atmosphere in place? In a typical sci-fi show, when a spaceship gets a hole, everything gets sucked out . . . but you had artificial gravity, would this actually happen? Could you have a convertible spaceship?
posted by fimbulvetr to Science & Nature (21 answers total)

No, because you're really talking about two different things here: gravity and air pressure. Unless you've got high enough gravity to prevent air molecules from achieving escape velocity, you're still going to asphyxiate. And if you do have that much graviational force, you're basically a squashed mess.
posted by le morte de bea arthur at 12:51 PM on February 24, 2012

Counterfactuals are conceptually slippery. As a body of mathematics, you can't a law of physics without altering the way everything works. If you don't try to maintain internal consistency, you might as well make up whatever answer you'd like to a hypothetical like this.
posted by phrontist at 12:54 PM on February 24, 2012

can't change a law of physics
posted by phrontist at 12:54 PM on February 24, 2012

I am not a scientist. I am not a sci-fi writer. I AM a sci-fi reader.

... Upon reloading, what le morte said. Air pressure, grav forces, squishy paste.

Heck of an idea, though. I suppose one COULD attempt to create an artificial planet with enough gravity at it's core to support a thin atmosphere. It'd have to be a pretty big construct, though, and you'd want to have it in place before trying to create the atmo.
posted by Heretical at 12:54 PM on February 24, 2012

Response by poster: That is one line of thought I had, but I was thinking that if the artificial gravity is enough to keep everything else in place, and gravity is what holds the Earth's atmosphere in place, why wouldn't an artificial gravity that simulates conditions at the surface of the Earth be enough to hold an atmosphere in place . . . .
posted by fimbulvetr at 12:55 PM on February 24, 2012

To simulate the conditions at the surface of the earth, you need to simulate the air pressure at the surface of the earth. And you can't do that by just applying gravity.
posted by le morte de bea arthur at 12:57 PM on February 24, 2012

Response by poster: Now what if your ship was basically designed like a bucket swinging around on the end of a rope simulate gravity . . . could you leave the top open?
posted by fimbulvetr at 1:01 PM on February 24, 2012

Nope. Centripetal force is good for simulating gravity, but it's no good at holding in gases. The pressure differential between the atmosphere in the bucket and the vacuum of space will such the air out of the bucket no matter how the bucket is moving... well, unless it's accelerating at an incredible rate, which again will squish the occupants.
posted by le morte de bea arthur at 1:06 PM on February 24, 2012

such = suck. Pardon my whisky fingers.
posted by le morte de bea arthur at 1:07 PM on February 24, 2012 [1 favorite]

I suppose if you had some sort of imaginary technology that allowed you to just exert forces out of empty space, you could have some sort of short-range force that would hold in an atmosphere. But that's basically the same as having a solid bubble around your spaceship.

Science fiction writers often resort to 'force fields' in situations like this. The term is so nebulous that you can have them do whatever you like, such as hold in an atmosphere, which is very handy if you want people leaping into fighter ships and blasting out of a mothership without having to wait five minutes in an airlock (which, let's face it, isn't terribly cinematic).
posted by le morte de bea arthur at 1:11 PM on February 24, 2012

Ringworld did that by having massive walls on each side of the ring to hold in atmosphere. But you'd need very tall walls in order to make it work. But for something like the starship Enterprise, you're looking at a pressure difference of ~15 psi between the interior and the exterior.

Which is why most science fiction involving spaceships sometimes includes some mechanism for dealing with hull breaches: airtight bulkhead doors or force fields are commonly used.
posted by CBrachyrhynchos at 1:12 PM on February 24, 2012

Assuming artificial gravity works the same as regular gravity (e.g. falls off at the same rate as distance increases) then your bucket starship, bizarre and heavy though it may be, could work.

SF writers who speak of ring-shaped constructions, e.g. Niven's Ringworld and Banks' "Culture Orbitals" speak of rings in space that use centripetal and very high walls to contain a pressurized atmosphere but are otherwise open at the top. The Culture has force (and other types of) fields at the core of their technology (which is always described as "field-based"), so it's not clear whether they cheat or not with a contaiment field. Orbitals are described in Iain Banks' essay about the Culture universe, A Few Notes on the Culture

I'm not sure I agree with le morte de bea arthur, but there you have it, argument and counterargument.

As for artificial gravity, well, we know that it doesn't exceed the limits of the ship which has it on board, (else you could safely walk atop the ship, and in SF, they never do without assistance), so we don't know what the actual limits of it are-- perhaps it never drops as a function of distance, and so on.
posted by Sunburnt at 1:14 PM on February 24, 2012

Your bucket would need to be deep enough to have atmospheric pressure at the bottom match that of the Earth. Over 53km to include both troposphere and stratosphere, I think? That's a big bucket.

You maybe could counteract the problem with a series of fans, but at that point, why not just build a roof.

If the artificial gravity is field-based rather than centripetal force, it does seem likely the same tech could be tweaked to make a force-field. I mean, gravity is a kind of force, right?
posted by RobotHero at 1:33 PM on February 24, 2012

You've got two competing forces for planetary atmospheres - gravity trying to hold everything in and pressure differentials trying to spread everything out (fluids don't like to be concentrated). For a planet, gravity wins in that tug-of-war out to the edges of the atmosphere.

If you have something like an open-topped spaceship, what happens? Well, assume your gravity plating exerts a force only in one direction, namely perpendicularly down "in to" the floor. Place a large ball of water on your gravity plate and what happens? The water spreads out to disperse as much as possible. Now water has surface tension holding in place, but air doesn't. The air will simply spill out over the sides. Some of it has enough momentum (remember that air is full of molecules constantly in motion!) to escape directly against the force of gravity, but it would mostly be like trying to balance a large water droplet on a slide plate without all that pesky surface tension and friction holding it in place.

If you put walls up, you'll trap some of the air but it won't trap all of it - air's simply not massive enough. At STP (standard temperature and pressure), the density of air is about 1.2 kg/m^3. According to Newton's law of gravitation, a cubic meter of air at sea level has about 12 Newtons of force acting on it. If you had that same amount of air on a gravity plate surrounded by empty space (let's assume it's now in a meter-cubed box with one open side so the inside should have standard atmospheric pressure), the differential pressure at that open face is 1 atmosphere, or about 101,000 N/m^2. The force across the whole face is about 101 kN, then - several orders of magnitude more than what your gravity can provide. Pressure differential wins and you lose all your air. To hold all the air in the box using only your gravity plate, it needs to exert a force equal to or greater than the pressure differential - 101 kN acting on 1.2 kg of air is about 8,600 g's.

The reason the walls work in these Ringworld thought experiments is that air lower in the atmosphere is pressed on by the weight of the air above it, holding it down. If you build your walls to the height where gravity and pressure differentials exert roughly the same force, then you don't lose any of your air.
posted by backseatpilot at 2:04 PM on February 24, 2012 [1 favorite]

Sure you could. You just need very fine control of your gravity generator. Think about those door-less refrigerators that keep cold air in behind a wall of laminar flowing air. Now if there were a gravity generator along the back wall of the fridge, you could tune it to exert an inward force (greater than the force generated over the interior of the fridge) that would tend to pull that wall of flowing air inwards. You then can pressurize the interior and the wall would hold. Since you don't really need earth sea-level atmospheric pressure at all (the space station and space suits are only like 3 psi or so) it seems entirely possible.

You have your space convertible with the interior space set to 1g, a thin-ish layer above that set to much higher g, and you pump a jet stream of air through that heavy gravity layer from the front and suck it up in the back. So really all you need it a configurable gravity field.
posted by zengargoyle at 2:56 PM on February 24, 2012

I think le mort de bea arthur is right.

A way of making his point more intuitive might be to think of what happens as you go down into the depths of the ocean.

At 3300 feet the pressure is ~101 times what it is at the surface, even though the force of gravity is almost the same, because the pressure is due to the accumulated pressure of the water and air above (not precisely weight/area, or the pressure at the center of the earth would be zero, because nothing weighs anything there).

In the same way, atmospheric pressure at the surface of the earth is due to the accumulated pressure of the atmosphere above (which is almost the same as the weight of the atmosphere above).
posted by jamjam at 2:56 PM on February 24, 2012

To answer the original question about a hole in the wall of a spaceship with artificial gravity, consider what happens when you make a hole in the wall of a vacuum chamber sitting on the surface of the earth. The air rushes into the chamber, with no significant effect from the real gravity pulling on it. Similarly air will rush out of a spaceship when the hull is breached, even in the presence of artificial gravity.
posted by monotreme at 3:39 PM on February 24, 2012

If one were able to create an artificial gravity like you see in sci-fi shows like star trek, would that be enough to keep an atmosphere in place?

Luckily, we have a real-world model for us to look at. It's called the Moon.

Even a body roughly one-sixth the size of Earth cannot hold any useful atmosphere.
posted by Cool Papa Bell at 3:51 PM on February 24, 2012 [1 favorite]

Response by poster: Lots of great answers. Looks like a spinning bucket ship with really really high walls or more hand-wavey gravitational/force field stuff are the only ways to build my space convertible. But hey, the bucket ship is theoretically doable.
posted by fimbulvetr at 8:00 PM on February 24, 2012

The Tardis has this feature.
posted by flabdablet at 10:09 AM on February 25, 2012

It's actually relatively easy to get an order-of-magnitude estimate for the height of an atmosphere. You know from thermodynamics that the average energy per molecule in an ideal gas is kT, or about 25 milli-eV. If one of these average-energy gas molecules happened to have a velocity pointing nearly upwards, and it didn't interact with the rest of the atmosphere on its way up, its path would trace a tall skinny parabola, topping out when all the kinetic energy has converted to potential, E = mgh. For a molecule with mass 30 amu or so, like nitrogen or oxygen, that's a height of about eight kilometers. Not bad for order-of-magnitude: that's about where the bottom of the stratosphere is, though the exponentially falling-off "tail" goes much higher than that.

So any room-temperature, nitrogen-oxygen atmosphere is going to "want" to be some tens of kilometers tall, with what happens at the boundary determined by more detailed physics.
posted by fantabulous timewaster at 3:46 PM on February 27, 2012 [1 favorite]

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