is it in the shade in outer space?
I'm sure someone more qualified to answer this will chime in soon, but I'd expect it to be around 3 degrees Kelvin, in other words the same temp as the cosmic background radiation.
That is much colder than I
thought. How do they prevent the
shuttle, say, from breaking apart
because of the temperature dif-
ferences between the sunny and
I believe it does creak a bit, as astronauts have reported. The ISS still more. This is a good point and thermal stress is one of the things the space engineers have to deal with.
Notice that satellites and space craft are mostly made out of thermally conductive metal. So they tend - only tend - to assume a thermal equilibrium, and that is actuallly pretty hot for objects close to earth's orbit. I believe the black body temperature due to the sun's radiation at earth distance is about 80o F.
I understand that skylab was always rather noisy because of expanding
and contracting of various parts---creaking, popping, drum-booming----the sounds that metal structures make under uneven thermal stress. Maybe they just allow for it in the design.
this is a fun question
and I am not offering an actual answer---only a discussion of it
I agree with radagast about 3 kelvin
but it makes one think about temperature and how its defined
(strictly speaking for systems in equilibrium, and yet
practically nothing is ever completely in equilibrium
so there is always a rakish ad hoc element in applications.)
and to extend the question of Zooby to the moon:
"how do they keep the moon from breaking apart
because of the temperature differences between
the sunny and shady side" which might be
one of those classic evocative questions
like "why is the sky blue"
There is always conduction laterally from the hot frontside around to the cold backside, or thru the body if it isnt hollow
but forgetting about conduction think about how a side of the moon which has been in the sun and now finds itself in the shade could cool off
what could cool it, since it is in vacuum?
only its own radiation-----so it gradually radiates off infrared.
The neat thing about this is that it follows an amazing fourth-power law discovered by two Viennese gentlemen of the Victorian era and half raised to the fourth is a sixteenth
so that after the temp has fallen down to one half of what it was, the thing is radiating away heat only a sixteenth as fast
and so it goes----slower and slower
when it is down to 1/3 of its original temp then
the rate energy is leaving it is down to 1/81 of
the original rate----very slow.
so once a thing has been exposed to sunlight and soaked up a little heat, if you put it in shade it will take a REALLY long time
to cool down to 3 kelvin----sitting in the vacuum with nothing
but its own radiation as a way of getting rid of heat.
this is just one thought evoked by that question
I could imagine people writing in with quite a few other
observations about it, and related questions like why
does the equilbrium temp of planets go as the square root
of the distance from the sun so that if a planet is 9 times
farther away its temperature is not 1/9 but instead is 1/3
as high, and stuff like that
Self Adjoint, I didnt see your reply when I wrote mine, could have simply not replied! Yes, you say something like 80F
What I find in a battered handbook is 394 kelvin for the flat surface facing sunlight at this distance from the sun
(the hot sidewalk on which one fries the egg, on the moon
to make sure there are no clouds)
and beautifully enough a generic black ball or let us say a cannonball of any size at this distance from the sun has an equilibrium which is
394 kelvin divided by the square root of 2
which is 279 kelvin
so I guess it would not be quite 80 F but more like 50 F (and my handbook can be a bit off) but both our estimates would be in the ballpark
numbers often frustrating, using 1370 watt per square meter as
solar constant and 5.67E-8 for StefanBoltzmann I actually
calculate 394 kelvin, in agreement with handbook.
But there is a bit of play in the solar constant and some people
use 1380, so they'd calculate something a bit higher for
the equilibrium temp
What's your guess on how close a
satellite gets to blackbody?
I think I'd jump right out of my
skin if I were on the shuttle and
heard it start creaking.
Fascinating stuff! It's comforting
to learn the moon won't be crack-
ing apart from temperature dif-
ferentials in the near future.
I completely blanked out on the
subject of air and was imagining
that as soon as a thing went into
the shade in outer space it would
be assaulted by unbelievable cold.
But of course, as you point out,
it isn't. All that happens is that
it begins to radiate its heat. It
is nice to find out that even this
becomes increasingly slower as it
progresses. I feel better about
the shuttle. I don't like the
sound of those Skylab noises, tho.
Zooby, to me the most aesthetic aspect
is a certain squareroot of two
that gets into the picture
in vacuum at any distance from sun
a generic flat surface facing sunlight reaches some equilibrium temp
(which depends on the distance)
and that temperature is the squareroot of two
times the temp that a generic cannonball object reaches
because the ball has four times the surface area
of a flat plate that is intercepting the same amount of sunlight
and because the fourth-root of four is the squareroot of 2
Pythagoras would have liked that, too bad no one told him
equilibrium temp depends on balancing incoming and outgoing
so it is determined by the fourthpower radiation law
Regarding sattelites and such, have you evernoticed that most sattelites spin along their own axis while orbiting? That's one way of deminishing the problems that come with unneven heating. Keep in mind, an object in orbit doesn't just have to deal with the thermal differences involved in going from the Sunward side of a body to its dark side; the object also has its own shade. The Shuttle and stations have "circulatory systems" to help cope. Thay continually circulate fluid around just under the skin, taking heat from one side to the other, while the craft is exposed to sunlight.
Yes, this huge difference between
the sun side and shade side of
an object in space is exactly what
got me wondering if it posed a
threat to the shuttle. Not that
the sun side would get very hot
but that the shade side was SO
dam cold that the temperature
difference would cause an
untenable contraction of the mat-
erial on that side.
Do you have any idea what fluid
they circulate, Lurch? What
earthly anti-freeze could with-
stand 3 Kelvin?
Yes, I had this notion that satel-
lights rotated on their own axis
but I could figure how they keep
their antennae oriented if they
rotate. Or cameras or reflectors?
My 80oF figure came out of an old Willy Ley book or article. He was pointing out that the problem in space stations (which did not exist when he wrote) was not keeping warm, but getting rid of heat.
And did that actually turn out to
be a problem? Given what marcus
brought up about the slow rate
of radiation it doesn't seem
Zooby indeed because of the slow rate of radiating heat
it is hard and even costly to dump waste heat
every energy conversion process, life, electric power generation,
even running a computer, produces low-grade waste heat which
if it builds up is unconfomfortable (and eventually worse than uncomfortable)
but at room temperature waste heat radiates slowly and so
you need a very large radiator surface
The good Willy calculated this using the fourth power law
both willy and selfadjoint are right. if I have said anything inconsistent with them I must have made an error.
I'm not sure what they use, I always assumed it was regular anti-freeze. Keeping in mind, of course, that the fluid never actually drops to 3o K; the problem is getting cool enough. As the fluid gets 'round to the cold side, it begins to radiate heat, but it never gets rid of all the heat it picked up on the warm side. So the whole sattelite maintains a (nearly) uniform temperature, and the fluid doesn't experience any real extremes.
As an example, I do know what fluid ciculates in an EVA suit to perform the same function. It's water. Or at least, it used to be, I'm not sure if that's what they still use.
For someone like myself who lived
in the frozen wastes of Minnesota
for eight years this all comes as
a counterintuitive realization.
It takes a bit of work to grasp
that without convection and con-
duction a Minnesotan could end
up being cozier on a space station
where the temperature outside is
three degrees above absolute zero
than he would just about anywhere
inside in Minnesota during the
This week in Space Weekly:
New McDonald's technology allows NASA engineers to "keep hot side hot; cool side cool"!
I have been thinking about this a little and nothing obvious strikes
me about Vienna during the reign
of Victoria that would induce two
gentlmen to start wondering about
radiation in the cold vaccuum of
space. What is their story?
Yes, in spite of frigid phenomena due to earth's tilt guaranteeing that its subpolar regions are shaded for months at a time, the earth is sensitive to waste heat radiation. Futurists predict that if all the third world were brought up to US standrds of energy usage the resulting waste heat would warm the earth by several degrees. This is in addition to the usual greenhouse sources of global warming.
Dont have time to research it adequately. Probably answer is in
some biography or history of science book. Here is some
background detail (but cannot answer main question):
Stefan was secretary of the Vienna Academy of Sciences from 1875 and discovered this thing about radiation in 1879. which then Boltzmann explained on theoretical grounds in 1884.
So Stefan was already a recognized Austrian science dude when he discovered the fourth-power law experimentally.
It was waltz time in Vienna and the Impressionists were
painting in Paris. Europe hadnt had a major war since Napoleon (going on 70 years)
Ludwig Boltzmann (1844-1906) was known for being something of a dandy and a playboy. He explained the second law of thermodynamics by the statistical behavior of atoms and molecules at a time when a lot of physicists refused to believe in atoms. As he got older he suffered from depression and eventually shot himself at some scenic vacation spot in Italy. It was not always easy to have as much fun as Boltzmann thought he should be having. He managed to lead a rather flamboyant life and be world-class creative theoretical physicist to boot.
Josef Stefan was an experimentalist. As far as I know he was just this Middle European guy who happened to measure the glow from hot objects and who happened to find out that if you made an object twice as hot it would radiate 16 times as much power.
It's very generic stuff, the hot object does not have to be in space. It can be an electric hotplate. It can be the sun. If the sun were twice the temp it is then it would make 16 times as much light.
Boltzmann, the bon vivant with the big beard, developed theory around this experimental fact. It became known as the
"Stefan-Boltzmann Law" or more formally as the
"Stefan-Boltzmann Fourth Power Radiation Law".
But Boltzmann did a lot else besides. He invented the fundamental physical constant known as "Boltzmann's k"
A lot of formulas in a bunch of different fields have a "kT" in them.
If you look at formulas for how a transistor works you see kT, or about heat capacities and melting points, kT, or how pressure and volume are related to temperature in gasses, kT, or the speed of sound, chemical reaction rates, whatall. If you looked up the formulas for how a star works inside, and how it is structured, they would have a lot of kT terms.
the unit in which Boltzmann's k is expressed is the unit of entropy,
or if you like to think of it another way, the unit of heat capacity:
amount of energy per degree of temperature.
Boltzmann's k gets into about as many formulas as Planck's hbar.
the Stefan-Boltzmann Fourth Power Radiation Law says the power (watts) that something radiates is proportional to the fourth power of its temp
power = sigma T4
where the constant sigma is a hairy combination of k, hbar and c:
sigma = k4/hbar3c2 multiplied by, of all things, pi2/60
Now this, I am afraid, is only background. Your question is how did they happen to investigate the relation between how hot she is and how brightly she glows, where she is a generic object, and how did they happen to find out this deep hidden proportion in nature. And why in Vienna.
You have to remember that since there is no air in space heat can only be carried away by radiation. So its not like the skin temperature of the shuttle is 3K.
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