Why don't we ever see an astronaut in a vacuum with a balloon? Is there any physical reason why don't we ever see an astronaut in a vacuum with a balloon? A balloon weighs so little, and so it wouldn't cost much to bring one into orbit, or even to the moon. They brought a feather and hammer to the Moon and dropped them together to show the absence of air resistance. They brought a golf club and two golf balls to hit golf ball very far, taking advantage of the absence of air resistance and the one sixth gravity of the moon. Why didn't the astronaut that dropped the feather also make a pendulum out of it by tying a thread to it? The other end of the thread could have been attached to a rod and the rod held aloft while the feather was whirled around in the vacuum.
Seeing a balloon being whirled around on the end of a string, and perhaps released into the vacuum would be a great physics lesson as well as extremely entertaining. I'm guessing that it would also have reduced the number of allegations that the moon landings were faked, but that's bye the bye.
If an astronaut could stand on something tall, say the lunar lander or a lunar hilltop, or mountain peak, and whirl a balloon around in a circle on a long light string below him or her, or perhaps at angle so it can be released at high speed, it would be remarkable and a lot of fun. How about throwing a 3m diameter balloon on the moon using a slingshot? Could it be thrown all the way to Earth?
The nature of a vacuum, the fact that there is a vacuum in space and on the moon, the nature of gravity, and the nature of balloons, and of buoyancy (especially if the balloon were filled with a gas like helium or hydrogen), and air resistance, would all be well illustrated. With a sufficiently light balloon, the pressure of sunlight could be observed.
By opening the end of the balloon, the fact that rocket propulsion does not depend on pushing against an atmosphere would be illustrated. It would be interesting to see the path followed by the open balloon in a vacuum. It would also be interesting to see whether a balloon bursts differently in a vacuum.
Is there some danger to this? Would sunlight destroy the balloon? If so, why not play with the balloon in the shade?
Would it be safe to do it in a vacuum chamber on earth? I haven't seen that either. In fact, I can't find a case of someone swinging or projecting a balloon in a vacuum chamber. Just a bowling ball and a very large feather being dropped together from a crane inside a giant vacuum chamber. So really this is a case of no balloon or feather or other object that has a large surface area to mass ratio ever being swung or projected in a vacuum. What could be the reason?
Could it be that it has happened but Google doesn't show the web pages that report it? That seems unlikely. Could it be that people who have done it do not report it, or at least not in such a way that 'knows' about it? That also seems unlikely. Could it be that no has ever done it? It seems unlikely, but it's less unlikely the other options (right?). If people have never done it, what could the reason be? That they simply didn't think of using a feather as a pendulum bob in a vacuum seems unlikely. That they thought of it but never did it seems unlikely.
I'm stumped.
Could an actual astronaut, or someone who supervises one, or who has access to a large vacuum chamber say why he or she has never swung a balloon or feather on the end of a thread in a vacuum? Is there a rule against that sort of thing? Is there a physical reason it can't or shouldn't be done?
 A: Valid suggestion, but there is a problem. You would need a really tough custom-made balloon to not burst in vacuum. Regular balloons burst at about 0.1 atm pressure difference, which means you will need a 10 times thicker, and therefore heavier balloon. So, experiments that rely on balloon being light wont work. Such balloon would probably have to be custom made, which is a lot of hustle. I am also not sure that the experiment would reveal something counterintuitive like in the hammer-feather drop experiment on the moon.
"Demonstrate absence of buoyancy in vacuum"? I never knew anyone who would think there is buoyancy in vacuum. If anything, you may want to show that there is buoyancy in the air (at some point in the past people thought didn't realize it), for which a regular helium balloon would do. "rocket propulsion does not depend on pushing against an atmosphere"? Does anyone think rocket propulsion relies on pushing from the air? How would rockets fly in space then?
To summarize, it is not as easy and the utility is questionable. I would even argue that it may create a negative reputation for NASA or whoever does this since the things the experiment is supposed to demonstrate are fairly obvious and intuitive.
A: A balloon in a vacuum would expand to more than its usual size as there is no counter-acting atmospheric pressure.
I agree it would make for an interesting and fun experiment. There is a famous experiment illustrating Galileos observation that all objects fall at the same speed no matter what their mass.  Here, an astronaut releases a feather and a hammer at the same height on the moon. They fall at the same speed and hit the ground at the same time this verifying Galileo's theory.
This experiment was recorded and viewable on youtube
The same experiment could be done with a balloon and a hammer with the same startling, to the eye, results.
A: Here is what I believe would happen:
Initially, the expansion of the gas (due to lack of pressure outside the balloon) would inflate the balloon to the point of bursting, see previous answers, but (assuming it survives its initial expansion) then the drop of temperature would shrink the balloon and freeze its surface into a brittle, shrunk solid.
Here is the example of balloons immersed in liquid nitrogen:
https://youtu.be/9vRMZSEF_a4
The difference with that experiment and my proposed thought experiment is that the experiment in the video is carried out at ambient pressure and not in a vacuum (so the ambient pressure adds to the shrinking of the balloons, in addition to the tension of the rubber balloon).
