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Comment by Pxtl | Hacker News Reader

That said, temperature works differently in hard vacuum - with no air to conduct/convect the heat away, isolated objects don't actually get "cold" the way they would in a pool of liquid nitrogen or something. Notice you don't hear about the ISS freezing at night.

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The ISS is only in shade for 45 minutes at a time, which is orders of magnitude less than a lunar night.

But speaking of that: how long is too long to spend at 26K in a vacuum? Are there any good articles/videos that talk about what happens to various materials at such extreme temperatures in a vacuum?

I think the point of the parent comment was that simply being in a cold vacuum doesn't make you instantly cold. That still requires a process of heat transfer, which occurs in a fluid via conduction, convection, and radiation.

When immersed in a vacuum the conduction and convection heat transfer paths are sharply reduced, so the question then becomes how fast do you cool off via radiation alone?

I'm not a physicist so I don't know, but it's certainly nowhere near as fast as conduction/convection (in fact I believe heat dissipation was actually problematic for many spacecraft designs).

As it turns out, even on earth's surface, radiation is a more efficient heat transfer method than conduction and convection through the air. Objects under a clear sky quickly drop below air temperature in many circumstances.

Thank you! I did an hour jog tonight, and most of that time was spent trying to figure out why there was frost on the ground with an air temperature above freezing and a warm breeze.

I wonder if heat might also be conducting downwards to the frozen ground below.

What he's trying to say is that if you toss a rock into liquid helium or a 4 Kelvin zero g vacuum it's going to get colder faster via direct contact with the helium than via radiation in the vacuum.

Wow, that seems amazing but now that you mention it I think I've heard of that happening even in deserts.

No, actually, a vacuum is an ideal environment for radiating heat away -- better than an atmosphere, certainly. Surfaces in lunar shadow really do drop to an extremely low temperature over a period of days.

You changed the entire context by sticking the word 'radiating' in there. Yes vacuum is fine for radiation, but radiation is much slower than physical contact with standard pressure air.

> Yes vacuum is fine for radiation, but radiation is much slower than physical contact with standard pressure air.

Actually, it's the other way around. Under a clear sky at the surface of the earth, radiation is much more efficient than convection and conduction to air. This is why objects at the surface readily cool below air temperature at night.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/coobod.htm...

The linked example is a person losing heat energy under normal circumstances (23 C ambient, body temperature of 34 C). The example shows that a body loses 17 watts to perspiration, 11 watts to conduction (to the air and environment), and 133 watts to radiation.

Contrary to what many people think, a planet without an atmosphere loses heat to space very efficiently, solely by radiation -- indeed, that mechanism is more efficient in the absence of an atmosphere. But even with an atmosphere, radiation is the major heat loss mechanism, most more efficient than convection.

A great deal of effort has gone into keeping the ISS at civilized temperatures:

http://science.nasa.gov/science-news/science-at-nasa/2001/as...

It's got 100,000 watts of solar power, so that combined with good insulation means it won't freeze as long as the panels keep working...

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