How high could a helium balloon go?

22 June 2008

Question

When I let my grandson’s helium balloon go up after his birthday we let it vanish into the sky. Could you tell me whether it would be affected by the very cold atmosphere if it reached 5 miles? Would it just pop or what would happen to it?

Answer

Firstly, the reason it floats up in the air is because helium is lighter than air. It's a bit light a boat floating on the water. The balloon is pushing air out of the way that weighs more than the weight of the helium and the balloon together so the heavier air comes in underneath the balloon and the balloon is pushed up in the air. That's why the balloon goes up in the air and this process will carry on going until the helium filled balloon reaches a point or an altitude at which the air is the same density as the helium + balloon effectively is. That is the maximum height which defines how high it can go.As you get higher up the air pressure reduces so the balloon is going to try and expand. Depending on how heavy the balloon is and how quickly it's losing helium (it can get through the gaps in the rubber polymers quite easily) the balloon will keep on going upwards until either it gets heavy enough it can't get any higher or the difference in pressure inside it and outside it is enough that it will explode. I think if you just let go of a helium balloon it will just escape and will go up a few miles. But we couldn't tell you exactly how many. Recently, Michel Fournier was planning to do a parachute jump from the edge of space a couple of weeks ago and unfortunately his balloon took off without him. His balloon would have got him to the edge of space because they use a material that can keep on expanding the farther up it goes, or by starting with the balloon almost entirely empty. There's just a tiny bit of helium in the top of them, so as they go up there's lots of space for that helium to expand into. They go up higher and higher. I think the altitude record for an unmanned balloon is about 53km.

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