1. JohnJones

    JohnJones New Member

    I think you will find that my description of the interaction of radiation with matter is in line with contemporary quantum theory.

    In the kinetic theory of gases, the temperature of a gas is defined as the average kinetic energy of the particles making up that gas. The atoms and molecules in the thermosphere being at a high temperature consists in the fact that they're travelling at high velocities. The only opportunities these particles have to convert their kinetic energy to radiative energy occur during collisions with other particles, which are rare due to their long mean free paths.

    You have formed the impression that the solar radiation in the thermosphere is particularly intense. But we can actually measure the intensity of solar radiation at the top of the atmosphere, where it's about 1.36 kW/m2, and compare it with the intensity of the solar radiation on the Earth's surface, where at midday it's about 1 kW/m2. (The Wikipedia article on "Solar Constant" is an accessible source for this.) That's not that big a difference, so it would be surprising if the radiative equilibrium temperature of a black body at the top of the atmosphere were much different than the equilibrium temperature of the same body at the Earth's surface.

    It would be interesting if you could answer Trailblazer's question about what makes the thermosphere special. Would you agree that the solar flux in the thermosphere is about the same as it is on the surface of the Moon, for example? If you think it's significantly higher, what do you suppose causes that?
     
  2. Hevach

    Hevach Senior Member

    For the thermosphere's temperature I've always liked the oven metaphor. Inside an oven at 400 everything is 400°. The air is uncomfortable but not particularly dangerous. The cake you just baked is too hot to handle but safe to touch for a few seconds at a time. The pan you baked it on will give you third degree burns almost instantly. The three do not transfer heat into your hand uniformly.
     
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  3. JohnJones

    JohnJones New Member

    This metaphor is helpful, but perhaps slightly misleading. Inside the oven, the radiative equilibrium temperature of a black body would be 400°, and if you stayed in the oven long enough, you would also be at 400° and get cooked. In the thermosphere, the radiative equilibrium temperature of a black body would be just above freezing point, and you could stay there as long as you liked.

    Our experience with the oven reflects the fact that air is a poorer conductor than cake, which in turn is a poorer conductor than the cake pan. But the anomalously high temperature of the gas molecules in the thermosphere has nothing to do with their conductivity, rather with the fact that, as isolated particles, they have no way to convert their kinetic energy into thermal radiation.
     
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  4. F. Serby

    F. Serby New Member

    I haven't read all of these, but has anyone mentioned the fact that the satellites also spend some time out of the sun's incoming radiation, when they are in the shadow of the earth? I don't know if all polar orbits (pole to pole) have an orientation that keeps them out of the sun's radiation--that would depend on what angle the orbit's plane is to the incoming rays--but geostationary orbits around the equator should have them in the shadow for a good percentage of the time they are in orbit, depending on the season.
     
  5. Layman research

    Layman research New Member

    This topic has interested me a long time. And this webpage just opened up my understanding exponentially. Thanks to all above for your input! Both skeptical and explanative! I'm taking all this great discussion about vacuums, radiative heat, relflectivity, it's great.

    The reason I started looking at this tonight is saw SpaceX Tesla roadster floating in space on YouTube, and opened the live stream chat... About half the chat comments were "fake.. It's fake." Then I also saw the rubber tires of the car, and thought "how can those not melt right up???"

    So I just found this website which states the average continuous temperature in space near the Earth is 248 degrees. Outer space, not the Thermosphere or Exosphere.

    I'm not someone who generally accepts everything I'm told, but I think through information and come to my own conclusions. So for things like flat Earthism or "all space flight and space exploration is a hoax" I disagree and tend to say... If you already are married to a conclusion, you'll probably keep finding reasons to keep together with that conclusion. (funny how that same mentality also drives climate change denial.)

    So here's the website, and I accept for now that satellites and Hubble and ISS are constructed with proper shielding and materials that can survive balancing out at around 250 degrees.

    https://sciencing.com/temperatures-outer-space-around-earth-20254.html

    Above near Earth outer space temperature is too simplified. Here's a fuller explanation from the referenced website :

    "
    "
     
    Last edited by a moderator: Feb 11, 2018
  6. Trailblazer

    Trailblazer Moderator Staff Member

    Again, space doesn't really have any temperature. Individual objects in space have an equilibrium temperature that depends on their distance from the sun and their reflectivity (albedo), plus any other effects (e.g. planetary atmospheres).

    For instance the Earth, with no atmosphere but the same albedo it has now, would have an equilibrium temperature of about zero degrees Fahrenheit (–1°F in fact). The fact that it has an atmosphere with a greenhouse effect makes the actual equilibrium temperature about 60°F higher than that.

    The tyres of the Roadster have a lower albedo than Earth, but they also have no atmosphere to warm them up. They won't "burn up", but the harsh unfiltered UV light will probably degrade the rubber pretty quickly.
     
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  7. Laser

    Laser New Member

    It may help to point out that cosmic rays with temperatures of millions of degrees are hitting your body constantly, but you don't melt because there are just such a tiny number of them that they can't heat you up enough to melt you before their heat dissipates.

    Another example is that you can wave your finger quickly through a candle flame and not get burnt. The candle is plenty hot enough to burn you, but not enough of the hot molecules in the flame are able to touch your skin to heat it up in the short time your finger is in the flame. It's true that a satellite stays in the heat plenty long, but there are vastly many fewer hot particles per volume in the thermosphere than in the candle flame. The thermosphere is so thin that it's kind of like waving one small hot candle under your spacecraft once every ten minutes and letting the heat dissipate in between.

    The high "temperature" radiation from the sun, such as UV and X-ray radiation is a different but similar story. If you look at all the radiation from the sun from all wavelengths, most of the total is from the visible and infrared radiation. Again there is just not enough UV and X-rays to heat an massive object in earth orbit up very much before the energy dissipates. Indeed, if you had some kind of filter that would block all the visible, IR, and lower "temperature" radiation from the sun from hitting your spacecraft, and only the high "temperature" radiation could warm your craft, not only would your craft not melt, it would get very cold because of the small amount of energy coming in from the high "temperature" radiation. I put the quotes around temperature because electromagnetic radiation doesn't really have a temperature, even though it may have a relatively high energy per photon.

    Some satellites can't naturally dissipate as much of their heat energy as they would like, and thus tend to overheat. In those cases radiators may be used. But sometimes even a plain radiator can't radiate enough heat away in the form of infrared radiation. So for extra cooling it may be necessary to use something like air conditioning. A refrigerating fluid is compressed, causing it to heat up. Then the hot fluid is pumped through the radiators to dissipate the heat energy by infrared radiation out to space. Because the refrigerant is hotter than the craft, it can radiate much more energy through a small radiator area than the craft at it's average temperature could. When the refrigerant is allowed to uncompress after dissipating much of its energy to space, it gets very cold and can be used to absorb the excess heat and cool the spacecraft.
     
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