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Why do helium-filled balloons float?
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this post was submitted on 10 Oct 2025
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The very short answer is that gas pressure is mostly proportional to the amount of particles per volume.
So a balloon filled with helium has X particles per cubic cm, while the air around it has the same amount (instead of getting crushed). But because helium is a lot lighter per particle than standard air, this makes the balloon lighter than air, and like trying to push an air-filled balloon underwater, this helium-filled balloon floats to the higher layers of air, until other smaller forces also start to matter and the balance is restored.
So a "vacuum-filled" balloon has nothing to give counter-pressure, but a balloon filled with helium definitely does.
I'll post this as a thread if it's beyond your scope to answer, but you seem like a smart person who might know a thing I have absently wondered about many times.
Balloons eventually fall back to the ground after the helium escapes. But let's say you made a theoretical balloon that couldn't allow the helium to escape, it's perfectly sealed. Now, let's say that the material it is made of is also as light as a regular balloon, but our near-magical material is also capable of withstanding extreme cold (like, outer space cold). Would it just float into space? I know that when we shoot rockets up there's a lot of heat from the friction, but if something is moving slowly, that wouldn't happen, I don't think? But can a slow moving thing escape earth?
Tell me, oh great and wise Heliumancer, tell me the secrets of the light-gas!
Close, but now you come into contact with the atmosphere not actually being the same density (in weight/volume as well as in particles/volume) throughout, but instead gets thinner as you get away from the earth.
For simplicity, assume space is actually empty, and the atmosphere gets thinner linearly up until x kilometers above sea level it's completely empty. Then the density will also decrease with height, and the helium balloon will eventually find a spot that matches its density, and stop there.
Again there's so much more to it but as a simplified model this works ๐
Rockets mostly need to fight speed (of the earth revolving around the sun), and indeed in our atmosphere speed means friction, but in space rockets still need a lot of propellant to change their trajectory. As always there's a relevant xkcd: https://what-if.xkcd.com/58/