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Let's assume that we build a giant steel hull in a shape of cube with open top (2km long edge) and lift it to the top of stratosphere and then pump air out of it. Would it float on the outer layer of stratosphere like a ship floats on the surface of water?

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You need to consider the pressure on the cube. The cube would have a certain initial average density (mass of the steel divided by the volume of the cube). The cube would sink to at least a level of its average density. Then the pressure would begin to crush the cube, decreasing the volume and increasing the density. The cube would progressively be crushed and fall to Earth, like this.

Also, if it is an open top cube, it would start filling with low density gas, increasing the average density of the cube.

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  • $\begingroup$ But isn't the pressure on this cube only a function of surface? Yet buoyancy would be a function of volume, so given sufficient size such a cube of certain thickness should withstand pressure and float or is this reasoning wrong? $\endgroup$ – Ardath Mar 28 '14 at 14:58
  • $\begingroup$ Pressure doesn't depend upon area, but the force of the pressure would be proportional to area. You need to compare the force to the strength and elasticity of the steel. A sheet of steel that size would be very flexible. Strength of the steel will not scale with volume. Think of railroad rail steel. Rail steel readily flexes to form the curves of a track, even though it weighs ~100kg/meter. $\endgroup$ – DavePhD Mar 28 '14 at 15:26
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Nice question! The answer is yes, it is certainly possible. There is no need to start at the top of the atmosphere though. The air around the earth has a density (and pressure) gradient that increases as you get closer to the surface, see the figure below. If you have an object whose average density is less than the atmosphere at the ground, then it will rise to a level where its density is equal to that of the atmosphere.

This is the same reason that, for example, oil and vinegar separate over time in a jar. It is also the reason that hot air balloons and blimps are able to fly.

Atmospheric Density Gradient

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  • $\begingroup$ The thing I was concerned about was that the surface of liquid is very clear, but the surface of gas layer is not and I wonder if such a lower pressure air from upper layers wouldn't build up in the lower levels of cube and eventually make it sink? $\endgroup$ – Ardath Mar 28 '14 at 15:00
  • $\begingroup$ Your observation is correct that an open top cube would probably not work for exactly the reason you suspect. The 'top' of the atmosphere isn't really a clear boundary. The International Space Station, for instance, is in an orbit low enough that the residual drag from the atmosphere requires it to be re-boosted periodically. $\endgroup$ – Chris Mueller Mar 28 '14 at 15:40
  • $\begingroup$ If you are willing to enclose some volume and hang below it, you can get pretty high. High altitude balloons have reached 53 km according to the Wikipedia article. $\endgroup$ – Chris Mueller Mar 28 '14 at 15:41

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