Sun and Cold in Space
Why is it cold in space when the sun's rays are traveling through the solar
It is not cold everywhere in space. Close to stars things get very hot. Remember that the
intensity of radiation falls off as 1/R^2 so as one moves away from a radiation source there
are fewer and fewer photons / meter^3, or in terms of waves the intensity decreases
In space, it is very easy to be cold on your left and hot on your right. The sun on your right
will only heat up your right side. Then your blood circulation would take that heat and try to
spread it around your body. Meanwhile all your skin, even the side towards the sun, would be
streaming out heat radiation in all the directions that are black.
If the sunlight you pick up has more watts than the heat radiation you emit, you get hotter.
If less, you get colder. Until the two match and balance.
The earth picks up 1000 Watts per square meter on the sun side, but it is loosing 300 Watts/m2
of infra-red heat radiation all around. All told, it stays at about room temperature. You
would too, if you were in "rotisserie-orbit" around the sun, and you
had the same lightness or darkness of color as the earth.
Closer to the sun, the balance is hotter, because dark sky stays the same, but the sunlight is
Farther from the sun, the balance is colder, because the sunlight is spread out thinner.
But if something stands between you and the sun, like your space-ship or the whole dead moon,
then you get nothing but cold dark sky.
This is a very interesting problem in space travel. Light rays only provide heat when they are
absorbed. Thus, in space where there is nothing to absorb the energy of the light it is very
cold. However, a space ship will experience two different environments. On the side towards
the sun it is very warm while on the side away from the sun it is very cold.
If the sun's rays travel through the solar system without interacting, they
do not leave any energy behind. If they do interact by, for example,
colliding with the earth, they are absorbed and do heat the earth, but
nothing further out. Because of the inverse square law the intensity of the
sun's rays rapidly diminish. A planet 10 times as far from the sun as the
earth would feel 100 times lower intensity than we do on earth.
We are very fortunate to be placed in an almost ideal location. The heat
absorbed from the sun's rays balances the heat radiated into space when the
earth is at a temperature well suited for life. Let us hope we remain lucky!
Best, Dick Plano, Professor of Physics emeritus, Rutgers University...
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Update: June 2012