Why does space cool a warm thing down so much?

[AluminumStudios]

Lets say you have a hot iron. It's molecules are vibrating a lot with a lot of heat energy which it gives off to the air molecules that colide with it which causes cooling. It also looses some energy in the form of radiant heat given off.

My question is, in space, why do things freeze (aka cool down) so quickly without the presence of air to help carry heat away?

When a warm object cools how much of a role does molecular collision with atmosphere play versus loss of heat in the form of radient energy?

Any insight is appreciated!

 

[Matthias99]

When a warm object cools how much of a role does molecular collision with atmosphere play versus loss of heat in the form of radient energy?

There are three ways to transfer heat energy from an object: conduction (direct contact with an object with lower temperature), convection (sort of a special case of conduction where the object is immersed in a moving fluid/gas with lower temperature), and radiation (all objects radiate thermal energy relative to their temperature, and absorb thermal energy from the environment relative to its temperature).

In an atmosphere, where there are lots of air molecules, convection/conduction dominates heat transfer. In a hard vacuum, where there are hardly any molecules to collide with, obviously it's the other way around, and radiation takes over. Most of 'space' is a giant sink as far as thermal radiation is concerned -- it's running about 4-5K. That tends to cool things off a bit. Of course, if the object is exposed to direct sunlight at close range, it'll fry pretty fast as well.

 

[91TTZ]

Originally posted by: AluminumStudios

My question is, in space, why do things freeze (aka cool down) so quickly without the presence of air to help carry heat away?

They don't freeze so quickly in space.

In space, things hold their heat much longer than they do on Earth. As another poster pointed out, on Earth you have conduction, convection, and radiation. In space you only have radiation.

That's why on spacecraft, cooling is such an issue because hot spots tend to remain localised. Satellites use peltier coolers to cool their circuitry and transfer the heat outside because they can't depend on air to carry the heat away.

A vacuum is the best insulator.

 

[CycloWizard]

The temperature of 'space' is typically considered to be around 3 Kelvin (-270°C, or -454°F). This means the driving force for heat transfer via conduction/convection (temperature difference) is very large. The lower rate of collisions (since space isn't a complete vacuum - there are molecules floating around up there, just at very low pressure), the thermal conductivity and heat transfer coefficients (proportionality constants between heat flux and temperature gradients) will be very low compared to in-atmosphere cooling.

Just as Matthias said, really, just not as well put.

 

[AluminumStudios]

Thanks for the replies, that makes a lot of sense.

I guess talking of keeping like Cassini's (Saturn) and Gallileo's (Jupiter) moving parts from freezing up and Hollywood confused my image of how fast things freeze in space.

 

[Calin]

If you have no incoming sun radiation (like behind the Earth), the radiation will work on the entire surface (or most of it) with a temperature delta similar to 200*C. This is huge, and will cool fast. However, in direct sunlight, the heat (or most of it) radiated on one side will be received on the other side. Have you kept your face turned to sun in a summer day? In the space, there is more heat received on unit of surface (as the atmosphere is not completely transparent).
this is one reason why space stations turn around - to have all the sides somehow heated (and not a side hot and one freezing cold)

 

[91TTZ]

Originally posted by: Calin
I
this is one reason why space stations turn around - to have all the sides somehow heated (and not a side hot and one freezing cold)

Space stations turn around? Not that I know of.

 

[Veramocor]

On this same general subject. How can you get nuclear reactors(not RTG's) to work well in space. Where do you dump the waste heat to? On Earth you have cooling water, in space you could only have a very large radiator.

 

[ahurtt]

Originally posted by: 91TTZ

Originally posted by: Calin
I
this is one reason why space stations turn around - to have all the sides somehow heated (and not a side hot and one freezing cold)

Space stations turn around? Not that I know of.

Ones in the movies do A donut or ring shaped space station that spins around not only serves to heat all sides of the station, it would also serve to create artificial gravity.

 

[Gibsons]

Originally posted by: Veramocor
On this same general subject. How can you get nuclear reactors(not RTG's) to work well in space. Where do you dump the waste heat to? On Earth you have cooling water, in space you could only have a very large radiator.

I think you answered your own question.

here's a review on space based reactors Text

 

[PlasmaBomb]

Originally posted by: Veramocor
On this same general subject. How can you get nuclear reactors(not RTG's) to work well in space. Where do you dump the waste heat to? On Earth you have cooling water, in space you could only have a very large radiator.

Technically nuclear reactors are cooled by liquid sodium and then water