# Can heat dissipate into a vacuum?

Asked by Fyrius (14419 ) June 13th, 2009

Does heat dissipation necessarily require a colder substance to absorb the heat?
If you’re in a space ship in outer space, do you need to worry more about heating or about cooling?

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Well, there is thermal radiation in the infrared spectra, but other than that I don’t know.

oratio (8837 )

How exactly do you think the heat from the sun gets to earth?

Lightlyseared (25641 )

@Lightlyseared
I’m not sure if it’s really heat that gets from the sun to earth. I remember people in high school trying to teach me that what the sun shines at us is radiation, which the earth turns into heat.

Can’t find any references for it. Does anyone else know what I’m talking about?

Fyrius (14419 )

Heat dissipates according to Newton’s Law of Exponential Decay. This does work in a vacuum, as heat transmission is through electromagnetic radiation, mostly infra-red. The heat ‘exists’ once that radiation is absorbed by atoms which then vibrate at a frequency that makes them feel ‘hot’.

That makes sense.

Fyrius (14419 )

All bodies above absolute zero radiate heat. The term you are looking for is Thermal radiation . This is a good description.

You can see that effect yourself when you use a large telescope outside. There are the normal thermal effects you’d expect as it cools to match the outside temp. However, if the conditions are right you will start to get dew on your equipment. That is caused by Radiation Cooling. The law of thermodynamics state that heat flows from hot to cold objects. The warm telescope is giving its heat to outer space. You need to use a dew shield to reduce the area of night sky that is robbing heat from your equipment.

Lupin (4354 )

Er, Lupin, you may want to check your physics. Dew on equipment is caused by the air temperature dropping, meaning that the air is saturated with water vapor that has to go somewhere. Pure convection is enough to explain this.

cwilbur (14072 )

@cwilbur Dew will form on any surface once it gets below the dew point. That part is well understood. The point I am talking about is the small but noticeable time when your scope cools off before other items of equivalent thermal mass and surface area. I’ll see if I can find some thermal data to show this. I recall reading about this effect in one of the astronomy site or space.com.

Lupin (4354 )

I found the article I mentioned above . We already know about heat transfer due to conduction and convection. I am specifically talking about radiation.
There is a neat experiment you can do to show this effect.
“A simple demonstration of this effect can be made by placing two similar surfaces out in an open field. Over one of them, by a couple of feet, place a sheet or blanket. The exposed surface will dew up easily and the protected one much later. This is because the one is protected from seeing the cold night sky. ”

“Conduction of heat to a surface in the case of a telescope is typically from the attached equipment. This might be the electronics and/or motors driving the telescope. Or, very typically with amateur telescopes, heat is applied from a so called dew zapper. The dew zapper is a heating element whose purpose is to raise the temperature of the telescope slightly. Generally it is applied at the mirror or corrector plate since that surface seems the most subject to dewing. The amount of heat has to be held to the absolute minimum necessary to stop dewing, and no more, since there are other deleterious effects of heating and raising the temperature of the telescope above the ambient air temperature. Many amateurs find this equipment not only effective but necessary, especially when the telescope is used in the open or in a roll off building where it is exposed to the cold night sky.

Convection is the transfer of heat from the air to the surface or vice versa. It is normally necessary to have the telescope very near the ambient air temperature to prevent convection currents from occurring and causing heat waves near the telescope objective/corrector plate. In order to increase the heat transfer from the air to the surface, the air must be moved. With good air motion, the telescope will move rapidly toward the ambient air temperature. This is good for several reasons. One is the reduction of heat waves just mentioned. Another is the reduction of the possibility of dew formation. If the surface is at the ambient temperature of the air, dew cannot form. (unless the dew point temperature is actually equal to the air temperature)

Radiation is a much more subtle way to transfer heat. When two bodies are at different temperatures, there will be heat transfer from the warmer body to the cooler body. The transfer is dependent upon the temperature of the bodies and the area that each body sees of the other. (It also depends on the nature of the surfaces in a complex way, but this effect will be overlooked in this discussion.) In the case of a telescope which is in an open setting, the telescope sees the entire sky and radiates heat to the entire sky. The effective radiation temperature difference between the telescope and the sky is quite large. The sky (atmosphere) has a temperature of about 200 Kelvin. The telescope, even at say 50 F, has a temperature of 10 Celsius, of 283 Kelvin. Thus the telescope will radiate more heat to the sky than it receives and it will cool off. If it cools just a few degrees, it might well drop below the dew point temperature. Then dew forms. All surfaces experience this same effect. We all know that any surfaces on an open viewing situation, plastic cases, chairs, wooden boxes, telescopes, everything gets wet with dew when dewing conditions are right. In fact, they get wetter and wetter and water will run and puddle on them.

There is a constant struggle between heat radiation from the surface to the sky and heat transfer from the air to the surface. The temperature of the surface will and must be below the ambient by some amount. Thus the surface can dew up. A dew shield on the front of the telescope will greatly reduce the tendency of the corrector plate to dew up because it greatly reduces the solid angle of the sky that the corrector plate sees. However, eventually the corrector plate will dew up if the heat transfer from the ambient air cannot keep up with the radiative cooling to the sky.”

It is awesome when you think about.

Lupin (4354 )

Answering the original Q: Yes, by electromagnetic radiation, although not by convection or conduction. Inside a sealed spacesuit losing heat only by radiation, the heat will probably build up to life-threatening levels, but, if it doesn’t kill you, the inevitable cooling by radiation, bringing your temperature down to the local vacuum background temperature (probably 3–5 drgrees absolute), will. If there’s also radiational input, the heat will just kill you sooner.

LexWordsmith (954 )

Latent heat, the heat we feel on our skin—no, as it is just kinetic energy that is transferred between atoms. Energy can be dissipated through radiation though. This can then be “transformed” into latent heat at a later time.

Ivan (13364 )

Isn’t it cold in space. That would suggest that there is no heat.

Blobman (500 )

@Blobman Not no heat, just little heat for most of it. The majority of the universe is around 2.5K. It will never reach absolute zero as long as there are stars shining somewhere in the universe within visible distance.

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