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OK. Hear me out. I know that blimps float because of the buoyancy of air, and there is no air in space. But think of the way a boat floats on the top of water even though the material that the boat is made of is generally much heavier than water. The amount of water displaced is heavier than the boat and all of its contents, so the buoyancy is sufficient to hold the top of the boat all the way above the surface of the water.

So is it possible to float a huge blimp on top of the ionosphere? An ordinary blimp is filled with the lightest air possible - hydrogen or helium, which maintains the structure of the blimp with its air pressure. But could such a structure be made so large that it could float on top of the ionosphere and be filled with nothing, so that it doesn't even need a top to keep out stray ions? (Maybe I should say space boat rather than space blimp.) Is there any material that could be light enough while still maintaining a structure that wouldn't leak? How big would it have to be to displace enough ionosphere to float up there?

And to preempt the obvious question, "Why?", I'm thinking in the same general direction as a partial space elevator. The energy cost of getting to space seems to be a primary barrier to space travel in significant volume. And every little bridge that we can come up with to lower the energy cost might put space within reach for a few more of us little earthlings. The edge of the atmosphere is only a small percentage of the way toward the altitude of geosynchronous orbit and beyond, where a space elevator could ultimately fling stuff into space. But the atmosphere is also where all of the turbulence will hit the structure, so maybe a support structure at that altitude could provide some marginally significant strength to such a structure. Imagine a space blimp/boat thing in a shape sort of like a Bundt pan with support struts extending to the elevator shaft.

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  • $\begingroup$ Essentially, your question comes down to displacement. Will the displaced weight of the thin air in the upper atmosphere take less volume than your Bundt pan? Sounds iffy. $\endgroup$
    – Don Branson
    Commented Nov 23, 2013 at 3:34
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    $\begingroup$ The idea to use buoyancy to get a rocket onto a higher altitude before igniting its engines isn't new. It's called a Rockoon and came up as early as 1949. The main problem of this technique is that it is extremely weather-dependent and thus unreliable. You never know where the balloon with the rocket will drift before it reaches ignition-altitude. $\endgroup$
    – Philipp
    Commented Nov 23, 2013 at 12:32
  • $\begingroup$ Actually, the question is more about material science. If we take the blimp to the very limit in size (one end in geosynchronous orbit and the other on the ground) we have what is called a space elevator. $\endgroup$
    – Aron
    Commented Feb 15, 2016 at 18:44
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    $\begingroup$ The primary flaw with this plan is that getting to space is much more about speed than it is height. Getting above the thicker atmosphere helps a bit, but most of the energy needed is to increase the speed to thousands of km/h to get into orbit, rather than just increasing altitude. $\endgroup$
    – Michael Richardson
    Commented Nov 30, 2022 at 23:05
  • $\begingroup$ Yes, if you could magically teleport your space ship 200 km above your head, without the ability to accelerate to orbital velocity (roughly 7 km/sec at that altitude) all you end up doing is falling 200 km to the ground. $\endgroup$
    – SafeFastExpressive
    Commented Dec 2, 2022 at 22:28

2 Answers 2

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Water has a well-defined surface. Everything below it is water, everything above it is air. The top of the atmosphere isn't like that; you just get a very gradual decrease of pressure until you're several hundred kilometers above Earth's surface.
You could try to build a cylinder with a closed bottom and open top, and place this upright, so it'll float like a bottle in water. But you'd have to build this cylinder hundreds of km high to prevent it filling up at the top, and strong enough to withstand the air pressure at the bottom because there will be a vacuum inside.
At an altitude of 50 km, the air pressure is about 16 mbar, so your structure would have to withstand 16 gram/cm^2.
The structure also has to be big, because the thin air has little buoyancy. At 16 mbar, air weighs 20 grams per cubic meter, so that's all it will support. It'd be very difficult to build a structure lighter than this that is strong enough to withstand the air pressure, let alone have it carry any useful weight.

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    $\begingroup$ "So you're telling me there's a chance." - Lloyd Christmas $\endgroup$
    – Mark Bailey
    Commented Nov 27, 2013 at 17:19
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    $\begingroup$ @MarkBailey: Pretty much there's no chance for a vacuum-filled structure (open or closed) to remain structurally stable and still light enough. 16 gram/cm^2 is 160kg/m^2 - imagine a 1x1m slab of - say, styrofoam, supported under the edges but not the middle. Make it a bottom of an aquarium, filled with 16cm of water - it must not break under the weight of the water. You'll need a slab some 10cm thick and it will weigh about a kilogram. You need 50m^3 of air for 1m^2 of the surface. 3/r is the surface:volume ratio for sphere; a 150m radius (300m diameter) just to break even without any payload. $\endgroup$
    – SF.
    Commented Feb 15, 2016 at 14:17
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    $\begingroup$ @MarkBailey: And it will need a payload: a vacuum pump to remove all the air/helium mix it was filled with during the production so that it wouldn't break apart under its own weight in ground level pressure. $\endgroup$
    – SF.
    Commented Feb 15, 2016 at 14:20
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An object floats (in water or in atmosphere) when it weights less than the water/atmosphere it displaces with its volume. That means the whole trick of building a blimp is building an object which has a lower density than air. The higher you get, the lower the density of the atmosphere, so you need an even lower density to ascend higher.

A vacuum has a density of 0. So when you could build an object which is only a vacuum with nothing to contain it, it would float till the edge of the atmosphere. But unfortunately this is impossible. You need something to contain the vacuum in. This container must be rigid enough to withstand the pressure-difference between inside and outside. Otherwise it would collapse, which reduces the volume. To provide this rigidity, it must have mass.

Interestingly, the amount of force the container needs to withstand reduces with height, because the pressure-gradient between inside and outside of the hull decreases. That means that after a certain height you could drop some reinforcements of the hull to reduce your weight and ascend even higher.

In the end, the maximum height you can reach through buoyancy is an engineering problem: Build an air-tight object with the lowest ratio of mass to volume possible. The better the ratio, the closer it will get to the edge of the atmosphere.

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    $\begingroup$ Why does the structure need to contain a vacuum? If it is filled with Hydrogen which is vented as it rises, the internal pressure could be kept equal with the external pressure, and it would be buoyant. I guess imagine a hydrogen-filled weather balloon with a vent to prevent it from breaking at very high altitudes. How big a balloon could you make, and how high would it rise? $\endgroup$
    – MattD
    Commented Apr 20, 2016 at 16:51

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