Great Mysteries #2

What's up with helium balloons?

Helium balloons have puzzled me since I was a small child. I actually have several unanswered questions about them. First, I've never understood the concept of helium becoming "tired" and causing three-day-old balloons to start coming down off the ceiling. This is patently absurd. Helium molecules are lighter than the oxygen and nitrogen molecules. What would cause them to acquire extra mass? Yet balloons do seem to lose their rising power over time, even when tied with a firm knot and without diminishing in size. Even if the plastic had microscopic pores allowing the gas to gradually drain, this would still make the balloon observably smaller. When I was in second grade, my class released balloons with messages attached to see if anyone would find them and write back. We were to release them on a Friday, but due to rain, we had to postpone until the following Monday. My teacher expressed concern that the helium, still compressed in tanks, would lose freshness over the weekend and our balloons might fail to rise when they were filled on Monday. So apparently helium becomes tired even when stored under pressure. (Or maybe my teacher was an idiot.)

I'm also curious and have never encountered an answer as to what typically happens to a balloon after it is released. How high does it reach at its highest point? What causes it to descend? (Tired helium?) At first thought, I would expect the balloon to burst at some point as the outside pressure diminished. After all, if you thoroughly inflate a balloon at the bottom of a swimming pool and release it, it will pop before reaching the surface. But my impression is that balloons released outside eventually make their way back down to earth. Perhaps the plastic of the balloon exerts enough tension to keep the helium compressed even as the air outside becomes less dense until it reaches an altitude where it achieves neutral buoyancy and then just floats around until its helium gets tired. If it weren't for weary helium, the balloon should theoretically stay up there indefinitely (or even just stuck to the ceiling). What about hydrogen? Does it get tired too?

And for a related question, how come balloons attached to the wall with static electricity invariably come unstuck after a few minutes? Surely the electrons don't become tired and lose their negative charge. A more plausible explanation would be that the negative charge is gradually dissipated into the surrounding air. So would they stay up if they were in a vacuum? (Obviously the balloons would have to contain far less air.)

Also related, on Farscape Rigel farts helium whenever he becomes nervous. How is this possible? (Obviously it is, because he does it.) Helium is an element and a noble gas. The atoms don't bind to anything, so they can't be extracted through digestion from food molecules (since they never occur in molecules). Rigel's food and drink may be laced with large quantities of helium. This is not entirely implausible. After all, here on Earth we carbonate soft drinks with carbon dioxide; they may do something similar with helium elsewhere. But Rigel eats the same food as his companions, and you never see them farting helium. The only other possibility is that his digestive system can somehow transmute one element into another. But that would be as implausible as the ability to poop gold bricks.

Comments

[schmed]

Hate to burst your bubble...

(Well, it was so easy.)

ANYway, the helium doesn't get tired, it gets away. Minor amounts of the helium *do* escape through the latex, but you can't tell visually because you can't detect the minor changes in the circumference of the spheroid. Additionally, the air pressure of day 2 is usually different (higher or lower) than day 1, so even if no helium escapes, the size would change. The reason helium-filled balloons rise so lazily is that they barely differ in density from the surrounding air when the mass of the ballon is factored in. If the difference was extreme, it would shoot up and try to bore a hole through the ceiling. That makes the loss of minor quantities of helium more critical to its relative buoancy.

The balloon will eventually keep rising until it pops. The more the outside pressure decreases, the more volume the helium displaces which makes it rise faster until the tensile strength of the weakest part of the balloon reaches the breaking point, then it returns to Earth.

If your sci-fi character ingests helium that exits rectally, his farts would always contain helium, nervous or not. They would also contain other gases ambient to his lower GI tract, so while the noble gas would not be traceable by one's olfactory organs, they'd still stink like shit.

[Tvindy]

schmed, thank you for that lengthy explanation. I was really surprised to read that the helium atoms can actually slip out through the latex, but that does make sense. Of course, your response has raised new questions in my mind:

--If helium balloons actually rise faster the further up they get in the atmosphere, how are they able to travel so far (given that they would pop fairly quickly)? Some of them have been recovered over 100 miles from the release point.

--Would filling a balloon just to the minimum amount required for lift enable it to go higher, since it would be under less pressure?

--Do weather balloons pop, causing their instruments to suddenly plummet back to the earth from thousands of feet up? Isn't that dangerous?

[schmed]

I neglected to factor in thermal layers and winds which also affect the inexorable upwardity. I don't know the equation on minimum fillings, but I bet the weather guys make it their business.

[ben]

as for the sticking to the walls I bet the negative ions get exchanged with the positive ions from the the wall and things become neutral again. Perhaps some exchanged with other particles in the air as well.

[Tvindy]

schmed, that's a good point. Perhaps condensation in the clouds also puts a little extra weight on the balloons.

ben, that's possible. Strangely, balloons stick to cats almost indefinitely.