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If a set of very large airtight pontoons were made in the vacuum of space, would they float on top of the upper atmosphere? I thought about this while watching the last season of Mythbusters when they crushed the train tank using a vacuum, and it reminded me a lot of what would happen if you depressurized a submarine under water. My thinking is that if the air pressure is enough to crush the tank, it should be enough to hold the tank up if it was placed on top of the air. My counter though is that air pressure increases too gradually from the upper atmosphere to provide an area that could stabilize the weight in a "floating position".

If they can float and support a significant amount of weight, it might be a good idea for launching spacecraft or low cost (after the cost of manufacturing) transport, as fuel wouldn't be needed to provide lift, just thrust.

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Yes, one could build very large structures in the upper atmosphere, but the economic advantages of such structures on Earth are not clear. The problem if launching into space is not that of providing "lift", the problem is to provide enough velocity. An air launch from a high flying plane only saves approx. 10% in total fuel due to atmospheric drag and altitude. If one could add 3km/s of initial velocity to a rocket, that would save the entire 1st stage, but it would require an enormously expensive electromagnetic launch assist device that could only pay for itself if we had to transport tens of thousand of tons of materials into space every year (which would create enormous problems with "space-junk"). Recycling the 1st stages is, for all practical purposes, the better idea, at least at the current level of space commerce. Having said that, your idea has long been considered as the most viable option for the initial colonization of Venus, since a 20% oxygen, 80% nitrogen mixture is actually a lifting gas in the Venus atmosphere and there happens to be a layer around 50-65km altitude where the atmosphere has essentially normal conditions. Your "pontoons" filled with air would be perfectly habitable while floating around the planet for decades.

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