MadSci Network: Chemistry
Query:

Re: what is the temperature in a vacuum?

Date: Thu May 26 14:03:38 2005
Posted By: Aaron J. Redd, Post-doc/Fellow, Plasma Physics and Controlled Nuclear Fusion, University of Washington
Area of science: Chemistry
ID: 1115327242.Ch
Message:

Hello Ed,

This is probably the last answer you wanted, but: "It depends".

As you know, the three processes for transferring heat energy from place to place are: convection, conduction and radiation. In our everyday experience, convection of the air is usually the dominant mechanism, and conduction typically dominates all the other cases.

In a vacuum you won't have convection, so what happens when the other two dominate? The answer depends entirely upon the surroundings: is the thermometer in contact with something very hot or very cold? What about the temperature of the surroundings -- is it all "room temperature" in every direction, or is one wall held at liquid nitrogen temperatures, or is there a red-hot filament nearby? And, if you do have facing surfaces at wildly different temperatures, then you can play games with the thermometer material itself (e.g., coat the thermometer with a substance that is very reflective for some wavelengths and very absorbent for others).

In the simplest possible situation (taking an ordinary glass mercury thermometer and putting it into a room-temperature vacuum vessel), the answer is that the thermometer may read cooler temperatures at first (because volatiles like water are evaporating off its surface), but it will ultimately settle back to the usual room temperature.

In a more complicated situation (suspend the thermometer by a wire in the vacuum vessel to reduce conduction, and then put a very hot surface and a very cold surface nearby -- like the hot filament and liquid-nitrogen-cooled wall mentioned above), the answer will depend upon the areas of the hot and cold surfaces, their absolute temperatures, the distances from the thermometer, and the material of the thermometer itself. That is, if you provide the set-up I just described, with a bare glass thermometer (or its equivalent), then you will get one result for the final temperature. Now wrap the thermometer in gold foil (good for absorbing short wavelength light, low emissivity of IR), and the steady state temperature will rise. Paint it white (nice and reflective for short wavelengths, dandy emitter of IR), and the steady-state temperature will lower.

You could look into the materials used in constructing satellites, probes and spacecraft: these devices will operate in the near-vacuum of space, with all heat transfer done by radiation. And the "background" is made up of space at a very low temperature, the nearby earth at a relatively balmy 250-300 Kelvin temperature, and the dazzling faraway sun at 6000 Kelvin. In this environment, different satellites will have reflective or radiative surfaces, depending upon the individual needs (how much power is being produced on-board, does the craft have a device that needs to be held at a particular temperature, is there on-board coolant, etc). You could probably find quite a lot of material by hunting around on Google. You might also get some information for free by bugging the Johnson Space Center, but I'm not sure who to contact at JSC.

In any case, I hope that this has helped. Good luck!

Aaron


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