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I have read these questions:

Why doesn't Helium freeze at 0K?

Do objects in space freeze immediately?

Now I do understand that a helium balloon cannot float up to space. But let's assume this balloon is already on a spacestation.

When they release it into space, two things can happen, it either pops right away, or it freezes.

Space is around 2.7K. So I assume helium should be liquid. But would the lack of outside pressure in space cause it to pop?

Question:

  1. What would happen with a helium balloon in space:

    1. would it pop ever?

    2. would it freeze?

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I think you may be misunderstanding what space being 2.7K implies/means. Before you release the balloon, in your experiment, you state it is inside a spacestation. At this moment, the balloon is at thermal equilibrium with the station, which means it is about 27°C (~300K) or whatever temperature the inside os stations are kept at. If you suddenly release the balloon into space its temperature will not instantly drop to 2.7K. In fact it would take a really long time for it to get to this temperature. This is because the way objects in space exchange heat with the environment is solely through radiation. Let me explain what I mean by this:

Every object that has a temperature radiates heat through electromagnetic radiation, in a way which is proportional to its temperature (if you want to know more about this search for Black-Body Radiation). This process of radiating means it is losing energy (heat) to the environment and is becoming colder. On the other hand, the object also absorbs the heat waves in the environment. If both the object and the environment are at the same temperature the object will absorb as much energy as it dissipates and will stay with the same temperature (this is what happens inside the space station).

When the balloon is released into space it will be in a situation where it is radiating more energy than it is receiving (because it is way hotter than the environment), so it will get colder and eventually freeze (but notice the freezing point of a balloon is way higher than 2.7K, so it can freeze quite fast).

Before this happens, though, the balloon will also expand, because the pressure of space is virtually zero, while the initial pressure inside the balloon is the same as the pressure inside the station. This pressure difference would probably make the balloon pop.

The same reasoning can also be applied to a balloon filled with any other gas.

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The balloon would pop first (assuming it's released from room temperature and pressure).

For helium to freeze, it must lose heat. There are three mechanisms for heat transfer: conduction, convection, and radiation. The first two don't apply because of the circumstances (nothing to conduct/convect to), leaving just radiation. The governing equation for radiation is the Stefan-Boltzmann law, which in turn depends on the Stefan-Boltzmann constant. You would have to work through the math (it depends on the area of the balloon, the amount of helium in it, etc) but the order of magnitude it takes for the helium to freeze is a few seconds.

Meanwhile the helium is still at $1$ atmosphere against what's almost a vacuum in space. Against this kind of pressure difference a typical balloon has no chance of staying intact. You can see from that question that a rubber balloon pops at a pressure difference of only $0.3$ atmospheres.

So the balloon pops before the helium freezes.

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    $\begingroup$ Isn't that small constant multiplied by the Kelvin temperature to the fourth power? For room temperature that's about 8.1 x 10^9 $\endgroup$ – DJohnM May 13 at 5:04
  • $\begingroup$ @DJohnM good point, the perils of simplifying things! I will edit the answer. $\endgroup$ – Allure May 13 at 5:08
  • $\begingroup$ Why do you claim that He would freeze by losing heat? It should still be a liquid even at absolute zero and low pressure. $\endgroup$ – thermomagnetic condensed boson May 13 at 18:05
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Helium when released from the space station will be at room temperature 300K.

Releasing this temperature and bringing it below the temperature where helium liquidifies only by the means of radiation will be quite tedious.

While on other hand there won't be much time before the pressure difference pops off your balloon.

The problem here is the rate of transfer of heat.

No method of heat conduction is fast enough to compensate for building pressure difference between internal and external side of balloon.

This is somewhat (sci-fi part)

If tomorrow someone would find a way to cool down your ballon faster (fast enough that it liquifies the helium before the pressure difference pops the balloon) he would be able to get the ballon out in space and liquify the helium before it will burst.

But what you will get then is a incredibly compressed ball of rubber that rarely qualifies as a balloon.

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I think "will the balloon pop" depends on how much helium you put in it.

A previous answer points to a report that balloons will pop with an overpressure of more than 0.3 atmospheres. But suppose I take a balloon that's designed to have a diameter of 30cm when fully inflated, and inflate it with helium to a diameter of 10cm (at standard pressure). If I put it in a vacuum, by the time it expands to 30cm diameter, that pressure will have dropped to $(10/30)^3$, or 0.037 atmospheres.

So, it's possible that the balloon will simply stretch until the tension from the balloon's material reaches equilibrium with the pressure inside it. After that, it will slowly cool via radiation (assuming there's no nearby source of radiation to warm it). Over a longer period, the helium will leak out; depending on the balloon's material, that might leave a shapeless wisp, or an empty, frozen shell.

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