Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kryptid on 05/06/2004 03:43:04
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I recently read on a website that a typical rubber, hellium-filled balloon can rise to about 28,000 ft. before it explodes. Here is my question: Assuming that a hypothetical helium balloon does not leak any of its gas, and does not explode, how high will it rise? Would it ever make its way into outer space?
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Helium floats because it is less dense than the surrounding air. In other words, for the same volume of gas, it is a lot lighter than room air and hence it rises. The same principle carries oil to the top of a puddle, hot air skywards, and keeps ships afloat at sea.
This is called Archimedes principle and it states that an object will feel a force equal to the weight of the fluid (or in this case gas) that the object displaces. So a helium-filled balloon displaces a greater weight of air than itself (because helium is lighter for the same volume of gas), and hence it feels an upward force and tries to rise.
The balloon will continue to rise until the weight of the surrounding air is equal to the weight of the helium (and balloon). But air becomes thinner (less dense) the higher you go, and therefore there is a theoretical maximum height that a helium balloon can reach. This occurs at about 20 miles above the Earth (that's just over 110,000 feet up, or three times the altitude at which most planes cruise). Since 'outer space' is judged to be about 600 miles above the Earth, the balloon cannot carry you into space.
The record for a human helium-driven ascent above the Earth is currently 113,000 feet. Last year 2 Englishmen attempted to break the record with a colossal 100-metre diameter balloon designed to reach an altitude of 120,000 feet. Unfortunately a set-back at launch prevented it ever leaving the ground. We covered this story on the radio when they announced their intentions the year before launch :
http://www.thenakedscientists.com/html/shows/2002.04.14.htm#1
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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28,000ft is about 8.53km... the boundary into official "space" is 100km up. Helium balloons rise because of air pressure around them forcing them up; they explode because the air pressure goes down the higher up you are in the atmosphere so they expand. So a helium balloon would be massive by the time it got near space, and there would be no more air pressure forcing it on its way... so it would just stretch the balloon until the rubbers elasticity balances the internal pressure of the Helium (which it would already of overcome by now exploding the balloon)... I think that’s all that would happen? Unless the balloon reaches some crazy escape velocity and shoots off into open space.
wOw the world spins?
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Thanks for the info. So a helium balloon can only reach about 110,000 feet or so? That's too bad. I was hoping that it might be of some help for getting into space more cheaply (attach a spaceship to a balloon for instance).
What about a balloon filled with hydrogen? Hydrogen is only half as dense as helium, so could it rise even higher, maybe even twice as high?
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Sorry Charlie, but it's been done already. Look here:
http://fly.hiwaay.net/~bbrown/rockoon.htm
The rockoon was an idea from the fifties, when multi-stage sounding rockets were too difficult or expensive. Balloon launches are tricky and unreliable, and range-safety considerations rule them out for most applications.
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Oh, I forgot, about hydrogen, just go here:
http://www.vidicom-tv.com/tohiburg.htm
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I realize that hydrogen is a dangerous gas to deal with, but how high can a balloon filled with it go? This assumes of course, that it doesn't leak, pop, or burn.
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Hydrogen has about 4 times the "lifting power" of helium, so a balloon could go very much higher, perhaps twice as high. OBTW, the Hindenberg was supposed to use helium, but the Americans wouldn't sell it to Germany because they were afraid it would be used in barrage balloons, ot other wartime uses.
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Hydrogen doesn't actually have 4 times the lifting power of helium, because the lifting power of gasses is related to the difference between the density of the gas and air.
in some slightly strange units the density of air is about 28, helium 4 and hydrogen 2
so helium's relative lifting power is about 28-4 = 24 units
and hydrogen 28-2 = 26 units
so you wouldn't get much higher on hydrogen, but it is a lot cheaper to get hold of as you can generate it from water, or natural gas, rather than having to find in the top of oil wells that are abover something radioactive - when alpha particles slow down they become helium atoms.
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Helium atoms have the mass of four protons (two neutrons plus two protons ) and go around singly because they are so unreactive (inert). Hydrogen atoms have a mass of 1 proton but have to go around in pairs as 'H two ' molecules. There's a basic law that tells us that equal volumes of gases at the same pressure contain the same number of molecules (Avogadro stated that two centuries ago). That's why Hydrogen is only half the density of Helium.
The 'lifting power' consideration is a real bummer. Looking on the bright side, though, a relatively dense balloon filling, like hot air, still does quite well for the same reason.
There is a great demonstration to show kids that "Air has weight". Get a big, rigid, container and a vacuum pump. Weigh it on a sensitive balance when it's full of air and then empty; you get a seriously measurable difference.