Temperature of Space: Atomic/Molecular Motion & Equilibria

[Frank Peters]

Temperature is atomic/molecular motion.

However, the temperature of planetary surfaces without an atmosphere are often quoted. For example, the temperature of the lunar surface ranges from about -200 F to +200 F depending on whether or not the surface is exposed to the sun or is in shadow.

But the moon has no atmosphere so then to what do these temperatures refer?

Are they the temperature of the actual ground surface layer in equilibium with solar radiation? That is, the gound surface layer absobs solar radiation and is heated but also loses heat through radiation back to space and conduction to lower layers. Eventually an equilibrium is reached. Is this the lunar temperature in daylight?

Also, putting quantum theories of the vacuum aside, can we talk about the temperature of interplanetary space? Or can we only describe a similar equilibrium phenomenon when a spacecraft is traveling through interplanetary space?

It would seem to me that interstellar space would be close to an infinite heat sink, as any object within it would radiate away all heat. Yet the voyager probes are still functioning.

 

[phyzguy]

Temperature is atomic/molecular motion.

However, the temperature of planetary surfaces without an atmosphere are often quoted. For example, the temperature of the lunar surface ranges from about -200 F to +200 F depending on whether or not the surface is exposed to the sun or is in shadow.

But the moon has no atmosphere so then to what do these temperatures refer?

Are they the temperature of the actual ground surface layer in equilibium with solar radiation? That is, the gound surface layer absobs solar radiation and is heated but also loses heat through radiation back to space and conduction to lower layers. Eventually an equilibrium is reached. Is this the lunar temperature in daylight?

The temperatures refer to the temperature of the lunar surface.

Also, putting quantum theories of the vacuum aside, can we talk about the temperature of interplanetary space? Or can we only describe a similar equilibrium phenomenon when a spacecraft is traveling through interplanetary space?

"Empty" space is filled with the Cosmic Microwave Background radiation (CMB), which has a temperature of 2.7K. So an object in interstellar space shielded from any stellar radiation should come into equilibrium with this radiation.

It would seem to me that interstellar space would be close to an infinite heat sink, as any object within it would radiate away all heat. Yet the voyager probes are still functioning.

The Voyager probes have their own onboard energy sources. These are Radioisotope Thermoelectric Generators. (RTGs). This heat keeps the probes warm enough so that they are still functioning. They are still generating significant power. This is what Wikipedia has to say on that:

"One example is the RTG used by the Voyager probes. In the year 2000, 23 years after production, the radioactive material inside the RTG had decreased in power by 16.6%, i.e. providing 83.4% of its initial output; starting with a capacity of 470 W, after this length of time it would have a capacity of only 392 W. A related loss of power in the Voyager RTGs is the degrading properties of the bi-metallic thermocouples used to convert thermal energy into electrical energy; the RTGs were working at about 67% of their total original capacity instead of the expected 83.4%. By the beginning of 2001, the power generated by the Voyager RTGs had dropped to 315 W for Voyager 1 and to 319 W for Voyager 2."

 

[anorlunda]

Temperature is atomic/molecular motion.

It sounds like you think that it takes gas or liquid to have motion.

Solids have temperatures too. You only need to touch a hot stove or a block of ice to know that.
The thermal motion of a solid's molecules can be compared to vibrations.

 

[russ_watters]

However, the temperature of planetary surfaces...

But the moon has no atmosphere so then to what do these temperatures refer?

Your premise is the answer to your question!

Yet the voyager probes are still functioning.

What does that have to do with your question?