Near Vacuum Thermodynamics
Name: Erin C.
Why would a person feel cold if they (theoretically) were
floating in outer space? I would have hypothesized that it would feel
warm, even hot, because space is a vacuum and would therefore insulate
conducted heat extremely well. You have answered a question similar to
this before (titled 'Why is space cold'), and left me with more questions
as a result. I realized, when I first thought of this question, that the
person must be losing heat through radiation. Therefore, we must be
giving off radiant energy right now, so why would a vacuum feel cooler?
What is the process of giving off or absorbing radiant energy - what is
actually happening in the microscopic level? Similarly, why is it cold at
the top of mountains? Is that related, if so, how? The idea of wind was
being explained to us in school, and we were told that when the hot air
rises, warmed through conduction by the hot earth, it cools and sinks
again. Where does the energy go, because we were also told that energy is
neither created nor destroyed? After reading your answer to the question
'Why is space cold', I researched black body radiation, and that has not
helped me understand this better. Thank you for your time.
Assume that you could float around in space without a space suit and without
contact to a space craft that could function as a heat exchanger. Under
those conditions, of the three modes of heat transfer -- convection,
conduction, and radiation -- only radiation is operative. The reason this is
so is that there is no medium, like air or water to "convect" or to
"conduct" heat away from you -- only radiation.
The temperature that you reach then is a balance of the heat you lose, and
the heat you gain by radiation. You will pretty much act as a black body,
and you can calculate the rate of radiation (at all wavelengths) that you
will lose. Your heat input by radiation will depend upon where you are in
space. The most dramatic example in near-space is the Sun, which could "fry"
the side of you facing the Sun while your back side freezes. Satellites in
Earth orbit are given a slow to equalize this effect. The amount of heat you
absorb is going to be a bit complicated, because it would depend upon
whether you are wearing a reflective coating, an absorption heat shield that
protects you from the Sun's radiation etc.
However, you fix these variables, there will be some temperature at which
the heat you lose by radiation equals the heat you gain by radiation. Your
steady state temperature is determined by that radiative heat balance.
It is true that a person in space cannot lose heat through conduction
(contact with other molecules). But likewise, a person in space cannot
receive heat through conduction either. In the atmosphere, we give off heat
through radiation, but we receive quite a bit through conduction. Because
of this conduction, the temperature of your skin will not go below the
temperature of the atmosphere around you. When there is no atmosphere
around you, there is nothing to keep your skin above a certain temperature.
In most places in space, it is cold. If the sun were shining on you in
space, you could heat up to 500 degrees Celsius. There would be no
atmosphere to absorb all the high energy radiation. The sun would feel MUCH
hotter than it does in the middle of July. If there is no heat source, such
as the sun, you can get as cold as -270 degrees Celsius. If the sun is
shining directly on you, you can get hot enough to fry to a crisp. The
atmosphere regulates the temperature around you.
Much of the heat in the atmosphere is radiated back into space. This is why
we continuously need the sun. Every day the sun replaces the energy that
radiates back into space. It is a continuous cycle. Energy comes in from
the sun, does whatever it does on that day, and then is released into space
(usually as infrared radiation).
Dr. Ken Mellendorf
Illinois Central College
Generally speaking, in space, you can assume that the
only method of heat transfer is radiation. The amount
of radiation received from the sun depends on an
object's position (e.g. is it in direct sunlight, is
it in a shadow, how far away is the sun, etc.).
That is how the object gets heated up. The object is
also, at the same time, losing heat via radiation.
So, whether the object is cold or hot depends on the
balance of radiation received or lost. In space, that
could be much in either direction, that is why things
can be very hot or very cold.
Hope this helps,
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Update: June 2012