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Name: Erin C.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 2001-2002


Question:
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.


Replies:
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.

Vince Calder


Erin,

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
Physics Instructor
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,

Wil Lam



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