Icy Hot Astronauts [duplicate]

Question

According to the specifications of the Apollo 13 lunar module where the astronauts were during their emergency return to Earth, the walls were about the thickness of a coke can so how were they able to maintain the temperature inside the craft to a degree that would be survivable during their 3 days in open space where the temperature is -300 °F or +300 °F depending on location and having to shut down almost all systems to reduce energy consumption? Wouldn't using any known device to regulate and maintain temperature consume vast amounts of energy especially with such thin walls and no protective insulation?

Answer 1

Common wisdom says that space is cold. I wouldn't go so far as to call that a misconception, but it's certainly a bit of a misunderstood fact.

See, space is not cold in the way your fridge or a block of ice are cold. Your fridge is cold in the sense that it's filled with cold air, which is in turn cooled by the cold back wall. But space just isn't filled with anything. (Or to be precise, it is filled with a phantastically thin gas or plasma, and that is actually very hot – millions of degrees – but because it's so diluted, it would also be wrong to say space is hot, as no macroscopic object would actually be heated by it).

So, space is rather “cold” in the sense of a businessman who passes by a beggar is “cold”: in the sense of not giving any warmth.

If you put a literal coke can in space, nothing much at all would happen at first. The walls stay at almost the same temperature as the liquid inside, because there's no air to cool or warm it from the outside.

The is another effect at work though: thermal radiation. On Earth, we only notice this for very hot objects, like a glowing piece of coal or the heating elements of a toaster. That's because at lower temperatures, radiation becomes much slower and air convection is then by far the more effective mechanism for transporting heat. But in space, radiation is the only mechanism that can transport from or to a spacecraft, so it is always the dominant one there.

As a result, your coke can will eventually cool out if it just hangs around in space. But especially if you first polish away the paint, this will take a long time, because it's not very hot to begin with and radiation scales down with the fourth power of temperature, and because reflective metals like aluminium emit especially low amounts of radiation.

And then there's the other direction of radiation, which we can also feel very well on Earth on a sunny day: the sun. A spacecraft in Earth vicinity will normally receive a lot of sunlight, and this also heats it, so much that many spacecraft have more of a problem with not getting too hot, than with staying warm. They use special radiators to remove excess heat, and again choose polished aluminium walls for the wals because these are also good at reflecting sunlight away and thereby preventing both excessive heating and cooling.

You could also use an insulator, like is done on many spacecraft, to keep the temperature stable. But that's to some degree unnecessary when a thin aluminium hull already insulates quite well, whilst being much lighter, more compact and less fickle. Also, in the long term you do need some active thermal regulation regardless. If the Apollo Lunar Module had been intended to be inhabited during the space journey, they might not have added thicker insulation but instead provided better heating through more fuel cells, solar cells, or simply dark-painted surfaces on the outside.

Answer 2

The temperature of an object/substance has to do with the average thermal energy of its atoms; the density of the substance isn't really involved. But for an object to change in temperature it needs to gain (or lose) enough total energy across all of its atoms to shift the average. Thus when a much denser object exchanges heat with a more diffuse one, the thermal energy gained/lost by one will equal that lost/gained by the other, but the temperature change each experiences will depend heavily on the density of each¹.

When the actual number of atoms is extremely low, as in the near-vacuum present in space near Earth, then even when the average thermal energy of the atoms in space around a spacecraft is quite high the total thermal energy is still extremely small compared to the amount of energy needed to raise the far far greater number of atoms in a pressurised spacecraft by even a single degree.

You've can demonstrate something like this effect in your own kitchen. If you turn the oven on to 180°C and leave it on you can get food nice and hot, because the oven is continually adding heat to the air to keep it at around 180°. If you turn the oven to 180°, let it get up to temperature, and then put the food in and turn the oven off, then your food will not get anywhere near cooked; even though the oven air is much hotter than the food (call it say 20°C for room temperature), food is much denser than air so there simply isn't enough thermal energy in the air to raise the temperature of the food by all that much. Of course, even though the air in your oven is far less dense than your food, space near earth is far far less dense again, so you can imagine how much less thermal energy there is available to transfer to/from the environment in space!

Thus the actual temperature of the region a spacecraft flies through doesn't really matter. Even though it can swing from being much hotter than the spacecraft (and thus thermal energy flows from space to the spacecraft) to being much colder than the spacecraft (and thus thermal energy flows from the spacecraft to space), the actual amount of energy flowing in either direction is simply inconsequential compared to other processes affecting the temperature of the spacecraft (such as absorbing radiation from the sun, humans and machinery producing heat as a by-product, losing energy as infrared radiation).

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¹ As well as the actual thermal properties of the materials involved, of course.