(I posted this on Clavius too.)

I used Stephan´s Law and found 120C for the equilibrium temperature of a thing absorbing an entire 1,400W solar constant. I think this matches the maximum temperature of the lunar surface, wich is a bad reflector.

When I tried the same thing for a 50% reflector, I found something around 60C. And for a 90% reflector, -50C. I found the first temperature consistent, but the second is colder than I would expect, and the third seems unreal. I supose the blackbody aproach may be too simple for those cases. What you think? What are the mistakes here?

(I suposed a surface of a bad conductor in vacuum, so I could neglect any energy loss for the backside of the material.)

When it´s said an astronaut suit reflects 90% of light, it means visible light, or the entire spectrum? What would be the reflectivity of a Hasselblad cartridge?

If it seems relatively easy to keep something cold just with insulation, why its seo common to keep satellites rotating?

I believe that the idea behind rotating a spacecraft is to minimize the temperature difference between one side and the next. This is why the astronauts referred to this as a "barbeque" roll, analogous to putting a pig on a spit and rotating the pig during a long cooking session.

(mmmmm barbeque pork)

I'm not sure whether you're doing the Stephen's Law calculations right. I think that in addition to compensating for the reflectivity (as you've done) you also need to compensate for the emissivity, which may not be 100%. (which is what you alluded to when you stated that the blackbody approach may be too simplistic.).

(I posted this on Clavius too.)

...where I answered it.

When it´s said an astronaut suit reflects 90% of light, it means visible light, or the entire spectrum?

All optical properties are functions of wavelength. You can, however, arrive at average or cumulative values through statistical techniques. But those values aren't necessarily useful for computation. It depends on who's telling you that reflectivity value as to what he means by it.

What would be the reflectivity of a Hasselblad cartridge?

Difficult to say since the magazine is actually coated with a coating that looks very much like what's put on older Thermos bottles. You need both the emissivity and the absorptivity; you can't reliably compute one from the other no matter what the simplifications tell you.

If it seems relatively easy to keep something cold just with insulation...

The problem is not strictly keeping something cold, but rather keeping it at a serviceable temperature, whether that means taking steps to cool it or to warm it.

...why its so common to keep satellites rotating?

Not all satellites rotate. The thermal design of a spacecraft is one of several design constraints that all go into the design pot and get solved one way or another for each specific task.

Communication satellites, for example, have to keep one face constantly facing the earth, meaning that they rotate with respect to the sun once in 24 hours. Other spacecraft rotate primarily for gyrostabilization, with added benefits in thermal management. The Apollo command module rotated because its physical structure was dictated more by aerodynamics than by thermal management. It had to operate in Earth's atmosphere, requiring materials that were not especially adapted to thermal management. The rotation in that case offered even heating, because if the heating is uneven you have mechanical stresses you don't want or need.

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Well,

My first impression was that the reflected energy would be the received energy times a constant (suposing they were talking about the same wavelenghts), and the emission energy would be just a function of the temperature, not the material (not such a thing like the "good" and the "bad" emissor).

Now I bet I'll have to dig some tables of physical properties of materials. Any ideas of how the correct calculation is done?

I'm not talking about complex cases. I know things like that may require computer simulations (I saw softwares about that; maybe someday I'll put my hands on one of them). For the start, I'm just talking about those ideal surfaces...

Wow, you're really determined to open the whole thermodynamic can of worms, aren't you?

Rather than type Thermo 101 into the board here, let me try to make sense of the higgledy-piggledy statements I've made here and elsewhere.

Kirchoff's law holds for average values over wavelength. But you have to consider that energy absorbed at one wavelength can be re-radiated at other wavelengths. So you have to distinguish between the laws that hold for specific wavelengths, the laws that are average in nature, and other laws (like Stefan-Bolzmann) that hold for cumulative (i.e., integrated) measurements.

The analytical computations are done just as you've done (when they're done analytically), but with actual empirically determined values for the optical properties, and using the Planck curves instead of scalar values. They often run to several pages.

For any real-world object, where you have to also account for interreflection, configuration factors, and so forth, you immediately go to the simulations or iterative solutions.

I know of thermal studies that have been done on the Hasselblad camera, but I haven't seen them. Presumably those studies will discuss the actual optical properties of the materials. The coating on the aluminum casings will be more important than the properties of the aluminum itself. I don't know exactly what the coating is, but it has the look and feel of other coatings that are selective emitters and absorbers.

__________________
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Clavius Moon Base

Jairo,

have a look at this.

It is a textbook about heat transfer, which goes beyond Thermodynamics 101.
The authors allow downloading the book for personal use.

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papageno


"Why waste time learning, when ignorance is instantaneous?" - Hobbes (Calvin and Hobbes)

"It's all about context!" - Vince Noir (The Mighty Boosh)

"I've never heard of such a brutal and shocking injustice that I cared so little about!" - Zapp Brannigan (Futurama)

Thanks Papageno - you've sorted out my Christmas holiday reading! (Oh, my wife will be so pleased! :wink: )

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I just read a text about the Hasselblad on the Lunar Surface Journal. It says the camera had a silver finish.

Some time ago, I asked why the camera used on Gemini 4 spacewalk were painted black. As that spacewalk lasted only some minutes, I think it really doesn't matter. But I read there were longer spacewalks in Gemini project that lasted several hours. I wonder if the cameras used there were still painted black. Do you know when they started to use the reflective ones?

(Thanks, Papageno. I'm reading the PDF now.)

Probably after the realised that the black coatings on the Gemini cameras were rubbish after a few hours? Remember, this was the first time any of this was done, so changes and improvements would have been the normal process during the development of things for Apollo. That includes the little things like cameras and defogging the helmet visors. There was probably a whole team whose job it was just to work on cameras and photography.

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When man started to take pictures with portable cameras in space?

I was thinking about Voskhod 2 spacewalk, but I read everywere it could be a soviet fake. Not that they weren't able to take pictures... but its not the best event to take as an example.

(Is there any info about the soviet cameras?)

I realize this is a completely different case in regards to thermal characteristics, but there is a report online that shows how the engineers for the TV camera predicted the thermal behavior of the camera and how the camera behaved in practice. See this page:

http://www.hq.nasa.gov/office/pao/Hi...sj-TVdocs.html

Specifically the report: Ground-Controlled Television Assembly:
Interim Final Report, RCA R-3838F, 25 February 1972

By the way, don't let the conspiracists know that the camera engineers built a model of the Hadley-Apennine region and lit it with a single 30 KW light to simulate lunar surface conditions!

They choose a material with a very low emissivity. Why?

Choosing a material with a low emissivity is not problematic. What one has to do is make sure that the ratio of absorptivity to emissivity is small, say less than 0.5.

There still is.

Missed this on previous looks through this thread.

The first author is the host of the radio series Engines of our Ingenuity and I had the pleasure of meeting him while working at the University of Houston. Dr. Lienhard graciously consented to give a talk on astronomical technology history when I was trying to come up with a replacement speaker for our astronomy society meeting. A most erudite gentleman, and a fascinating speaker. I didn't know his son was also an engineering professor.

We used to talk about putting satellites into a "rotisserie mode", which was very similar, but had an additional purpose. If a satellite screwed up, then "safe mode" involved putting it into a slow tumble, so even if the ACS was throwing a wobbler, the solar panels should spend at least some time pointing in the direction of the sun and generating a bit of power to keep it going until it could be recovered. (It also helps to have an antenna spending some its time pointing roughly in the direction of the earth.) If you didn't do this, then there was always the possibility that the ACS (and Sod's Law) would leave it pointing in exactly the wrong direction.