Is space an insulator? Why do satellites stay warm?

[dotancohen]

I have heard it said that although the temperature of space (NEO) is very cold, the vacuum is actually an insulator and therefore satellites have problems overheating. This is the reason given that spy satellites cannot be painted black, and therefore be made harder to see.

However, in the movie depicting the events of Apollo 13 we see the crew freezing. So which is it: does space provide insulation and the problem is overheating, or is it cold and the problem is freezing?

Thanks!

 

[Integral]

Everything (including the Earth itself) sees space at ~3K and loses heat via radiation to that temperature.

This is one reason why all satellites are rotating, the side facing dark space lose heat at a much higher rate then the side facing earth. Rotation means that a uniform temperature distribution can be maintained.

It is my understanding that even on a warm summer night you can direct the open end of a thermos bottle to dark sky and freeze water in the bottle. This leads up to the answer to your question about the insulation capabilities of a vacuum. There are 3 major mechanisms for heat transfer Conduction, convection, and radiation. If you surround a body by a vacuum you eliminate all losses by conduction and convection which leaves only radiation as a mechanism for heat loss. So yes vacuum is a very good insulator.

 

[russ_watters]

Space doesn't really have a temperature. The oft-cited temperature of ~2K is the black body temprerature of the radiation traveling through it.

There are three ways to transfer heat: conduction, convection and radiation. Since there is no air in space and nothing touching a satellite, space provides perfect insulation against conduction and convection. So all heat transfer is via radiation.

For a satellite, there are three issues: receiving radiation from the sun, receiving radiation from (or emitting to) the Earth and emitting radiation into space. The balance of these determines the temperature of the object. There is also insulation against radiation in the form of shiny metal foil, which reflects the sun's thermal radiation away and the satellite's thermal radiation in.

But if a satellite needs to be very cold (like a telsecope) or produces a lot of its own heat, the energy balance can be a problem.

So both or either too hot/too cold can be a problem.

 

[Phyisab****]

It is my understanding that even on a warm summer night you can direct the open end of a thermos bottle to dark sky and freeze water in the bottle.

Really? Can you show the plausibility of this with some simple calculations? i.e. show that the conductive and convective heat into the thermos is less than the radiative heat loss.

Maybe I'll think about that when I graduate in 3 days.

 

[dotancohen]

Really? Can you show the plausibility of this with some simple calculations? i.e. show that the conductive and convective heat into the thermos is less than the radiative heat loss.

Maybe I'll think about that when I graduate in 3 days.

Perhaps even more amazingly, googling it (now, less than an hour after the claim was posted) returns this tread as the first search result!

 

[dotancohen]

For a satellite, there are three issues: receiving radiation from the sun, receiving radiation from (or emitting to) the Earth and emitting radiation into space. The balance of these determines the temperature of the object. There is also insulation against radiation in the form of shiny metal foil, which reflects the sun's thermal radiation away and the satellite's thermal radiation in.

This, combined with the knowledge that the Apollo 13 astronauts were freezing, implies that the spy satellites have some very hot-running equipment on board, or are receiving much energy from their environment (solar panels?). Assuming that it is hot-running equipment, that would mean some powerful energy source on board, an RTG maybe? That just doesn't sound plausible.

I am familiar with the concepts of conduction, convection and radiation and I agree that only radiation would be an issue here. Well, that is part of the question. I suppose that it boils down to:
1) Is space enough of an insulator where convection is not an issue?
2) Would a mostly-white spacecraft radiate more energy than it would receive from the Sun?

 

[Integral]

First compute the energy lost to space = emissivity * Stef Boltz * Temp difference

This assumes a thermos with a opening of 1cm radius. Water has an emissivity of .99. and that the water is already a 0, so actually the losses will be higher, since it starts at say 20C.



Here are the numbers I used.

Stef.Boltz 5.7 E-8 $\frac J {(m^2 s K^4)}$
emissivity 0.99
radius 0.01 m
Area 3.14 E-4 $m^2$
Twater 273K
Tspace 3K
Tdiff 287K
Tdiff^4 5.314E+09 $K^4$
Heat loss/s 9.42E-02 J/s

hr 8
sec 29280
2.759E+03 J heat loss over 8hr


Now compute how much energy per g to freeze water.
heat of fusion 334 J/g
4.186J
Water temp 20C
m(ct+h) 4 417.72 J/g


conclusion:
If you could point your bottle at a sufficiently dark part of the sky you should be able to freeze about 4 or maybe 5g of water. I have left some room for environmental losses. Since you want to keep it pointed at a dark place you will need a clock drive, different problem.

The more light that enters the bottle the higher the temperature you are radiating to thus the lower your losses. If the moon was within view from the bottom of the bottle you certainly would not freeze the water.

It has been a long time since I have done this sort of calculation, hope I got it all right. Note that the opening radius was just pulled out of the air, seems like a reasonable value, if it gets to big then the environmental loss will increase, smaller you lose less energy.

1) Is space enough of an insulator where convection is not an issue?

For the third time neither convection nor conduction are present in space.

2) Would a mostly-white spacecraft radiate more energy than it would receive from the Sun?

Color is not what determines whether a object adsorbs or emits energy. All that matters is the temperature of the object. If a surface of the object can see the sun, as long as it is at a lower temperature then the sun it will adsorb energy. If a surface of the object can see deep space it will lose energy if its temperature is above 3k. If it can see the Earth it will either loss or gain depending on whether it is colder or warmer then the Earth's surface temperature. This is why satellites are rotating, to ensure that one side does not overheat while the other freezes.

 

[dotancohen]

> For the third time neither convection nor conduction
> are present in space.
>

I had thought that these were not an issue even before posting. I reiterated them there to clarify the possible issues.

> Color is not what determines whether a object adsorbs
> or emits energy.
>

Really? I really did think that black objects absorb more energy.

By your arguments, the Apollo 13 craft should not have frozen as was seen in the movie. Surely the craft was at a lower temperature than the Sun which was visible, and also the Earth while not being hotter, would not have been freezing either. So long at there are stars visible from all directions, the "empty space" that is not Sun, Moon, or Earth as visible from Apollo would not have frozen the craft, either, as per the water-in-a-thermos example that you state.

So what did cause the craft to approach freezing temperatures, or was that just a plot device added by Hollywood and not a real issue on Apollo 13?

Thanks!

 

[D H]

Post #3 by russ_watters was a good response. After that, not so good. The best thing to do is to start over.

I have heard it said that although the temperature of space (NEO) is very cold, the vacuum is actually an insulator and therefore satellites have problems overheating. This is the reason given that spy satellites cannot be painted black, and therefore be made harder to see.

However, in the movie depicting the events of Apollo 13 we see the crew freezing. So which is it: does space provide insulation and the problem is overheating, or is it cold and the problem is freezing?

The answer is that both overheating and overcooling can be big problems in space. The Sun is a huge external heat source, and spacecraft typically have computers and other components that generate waste heat. The side of a spacecraft facing the Sun can get very hot while the side facing away from the Sun can get very cold. A number of techniques, some passive, some active, are employed to ensure the spacecraft (or at least its temperature-sensitive components) are at the proper temperature.

Passive techniques include use of special coatings and paints, thermal blankets, sun shields, and thermal radiators. Active techniques include cooling systems that transfer waste heat from the spacecraft proper to thermal radiators, rotating the thermal radiators so as to minimize the area exposed to the Sun, and rotating the vehicle so as to minimize thermal stresses in the vehicle (barbecue mode).

Contrary to post #2, all spacecraft do not rotate, and of those that do, they do so in order to properly perform their primary mission (e.g., observe the Earth). Contrary to post #7, coloration (or rather coating) does matter. It changes the absorption and emissivity of the spacecraft .

 

[Integral]

Note that I made NO reference to amount of energy transfered, indeed color has a large effect on that. It remains the temperature difference which determines the direction of energy flow. That is all I was addressing.

 

[D H]

So what did cause the craft to approach freezing temperatures, or was that just a plot device added by Hollywood and not a real issue on Apollo 13?

The Apollo spacecraft thermal control systems were designed to maintain a proper temperature balance assuming a certain thermal load. The Apollo 13 crew had to shut down almost all of the electrical equipment on the spacecraft . The assumed thermal load did not exist. The spacecraft got very cold as a result.

 

[dotancohen]

The Apollo spacecraft thermal control systems were designed to maintain a proper temperature balance assuming a certain thermal load. The Apollo 13 crew had to shut down almost all of the electrical equipment on the spacecraft . The assumed thermal load did not exist. The spacecraft got very cold as a result.

Thank you. This is the most plausible explanation that I have heard. Might I ask from where this is known? I would love to read whatever informative article provided that insight.

Thanks!

 

[russ_watters]

Thank you. This is the most plausible explanation that I have heard. Might I ask from where this is known? I would love to read whatever informative article provided that insight.

Thanks!

What part are you looking for confirmation of? The fact that they had to shut down most of the spacecraft systems is common knowledge info you can get from the wiki or even the movie. The rest is what you know from heating your own house!

Based on this link, it looks like the nominal power output of the Apollo CSM was about 3kW and with an efficiency of perhaps 60% that means 5kW of dissipated heat. That's enough to heat a small apartment on a pretty cold winter day.

http://www.nasm.si.edu/exhibitions/ATTM/a11.jo.fc.2.html

 

[D H]

Thank you. This is the most plausible explanation that I have heard. Might I ask from where this is known? I would love to read whatever informative article provided that insight.

Thanks!

From my head. I work with spacecraft for a living. That said, I double-checked with various NASA sites to confirm that what I said was correct. The main thermal problem on an Apollo mission that went anywhere close to according to plan (none went perfectly) was getting rid of excess heat. Because of this the Apollo Command and Service Modules had a good-sized active thermal control system. Passive thermal control was aimed at minimizing solar heating.

This page, http://history.nasa.gov/alsj/CSMNewsRef-Boothman.html contains a lot of details about the Apollo spacecraft . The descriptions of the Command Module, Service Module, and Environmental Control Subsystem are quite pertinent.

This document, http://www.hq.nasa.gov/office/pao/History/alsj/a13/a13-techdebrief.pdf, is the crew debrief of the Apollo 13 mission. One new thing I did learn: The crew closed the shades on the windows so they could sleep at "night". This turned out to be a mistake acknowledged by the crew. It made the capsule get even colder.

This document, http://www.hq.nasa.gov/office/pao/History/alsj/a13/A13_MissionOpReport.pdf, is the Mission Operations Report. You can read after-the-fact reports by various mission controllers on how they addressed the problems that popped up on Apollo 13 and how they solved them.One thing I could not find was a reference as to why it got so cold in the module. The reason was rather obvious to me, so I suspect it was blatantly obvious to the people working the mission. They probably didn't think it needed to be explained: Why explain the obvious? Of course it isn't obvious to someone who does not work with spacecraft day in, day out.

 

[dotancohen]

What part are you looking for confirmation of?

Though to myself (a layman) the idea that the spacecraft cooled because it's electrical systems were powered down seems plausible, I wondered if it was conjecture on the part of DH or if that was the official position. Also, I figured that any source which would provide that information would be an interesting read for other insight that it might provide. I meant to express no doubt, but rather to enjoy reading the document from where that knowledge came.

Thank you DH and Russ for the last two very informative posts. There is much interesting material there and I am reviewing much of it now.