The typical speeds of helium atoms in the atmosphere are less than escape speed. But there are tails to the distribution: There are always some individual molecules or atoms that are going faster or slower than that, and some of the atoms out in the high-speed tail are going fast enough to escape. So those do escape, and the ones that are left interact with other atoms and re-form a thermal distribution, which again puts a few atoms out in the tail to escape, and so on.
Actually, this happens with all gases, but it happens more quickly with helium and hydrogen, since the velocities are higher for them.
Chronos: OK. The dudes in the ISS–breathing air, naturally, or they would have other things to do than fuck around with balloons – release helium-filled balloon at some point where the balloon is not accelerating. Which way does the balloon go? Nowhere, I guess.
As the lady said, never mind.
But when say, a helium/hydrogen- filled balloon is in some healthy stationary orbit or Lagrangian point (which I’m now learning about, thanks to Chronos) wouldn’t the atoms stratify in such a way as to cause the mass distribution within the balloon to fluctuate, which would imply…I have no clue…
And how dare you not notice my previous post. I’m hurt.
Ah, the five whys… I think it’s all about atomic weight. Helium, with atomic weight of 4, following only hydrogen. Four fundamental forces; strong and weak nuclear, electromotive, and gravity.
Density. Viscous flow; molecules collide with each other before vessel walls; gases; less dense. Just as oil will lay on water, or a coin on mercury, heavier elements are thus more affected by gravity; nitrogen and oxygen are confined to ground level due to their atomic weights being ~four times heavier than heliums; Hydrogen and helium are going to rise above them…
I like this question; will a helium balloon rise in a vacuum? I think pulling the balloon from 760 torr, into less than a hundred millitorr would likely burst the balloon; however, When I pull a vacuum in a vacuum bakeout oven, there is no effective convective heat as we pumped out the nitrogen, oxygen and argon, all of which have higher atomic mass than helium. So, even if the helium balloon was intact, I don’t expect it would float. The force of gravity is still alive and well, so I expect the balloon would sit in the bottom of the chamber. Incidently, the only heat we get in vac bake ovens is radiant and conductive.
I asked the same question of a bunch of physicists once. The answer is easy if the helium is in a balloon. You can calculate the pressure at the top of the balloon and the pressure at the bottom and see that the balloon will be pushed upward by the difference in pressure.
The question gets harder without the balloon. If the helium is free (imagine the balloon breaks) it still gets pushed upward, but why?
One of the physicists suggested a thought experiment with a bunch of marbles in a vibrating frying pan. I can’t remember the exact details. Some small marbles (the helium) and some large, denser marbles (other gases of various kinds). As the marbles are vibrated the smaller, lighter ones will tend toward upward or, more precisely, the heavier slower marbles will displace the lighter ones as they are pulled downward by gravity.
