Especially significant is the effect the absence of air exerts on the thermal conditions of outer space. Because as we know today heat is nothing more than a given state of motion of the smallest material particles of which the materials of objects are composed, its occurrence is always associated with the supposition that materials exist in the first place. Where these materials are missing, heat cannot, therefore, exist: empty space is "heatless" for all practical purposes. Whether this is completely correct from a theoretical standpoint depends on the actual validity of the view expressed by some experts that outer space is filled with a real material, distributed very finely, however. If a total material emptiness exists, then the concept of temperature loses its meaning completely.
This view does not contradict the fact that outer space is permeated to a very high degree by the sun's thermal rays and those of the other fixed stars, because the thermal rays themselves are not equivalent with heat! They are nothing more than electromagnetic ether waves of the same type as, for example, light or radio waves; however, they have a special property in that they can generate, as soon as they impact some material, the molecular movement that we call heat. But this can only happen when the waves are absorbed (destroyed) by the affected materials during the impacting, because only in this case is their energy transmitted to the object and converted into the object's heat.
Thus, the temperature of a transparent object or of one polished as smooth as a mirror will only be slightly elevated even during intense thermal radiation. The object is almost insensitive to thermal radiation, because in the first case, the rays are for the most part passing through the object and, in the latter case, the rays are reflected by the object, without being weakened or destroyed; i.e., without having lost some part of their energy. If, on the other hand, the surface of the object is dark and rough, it can neither pass the incident rays through nor reflect them: in this case, they must be absorbed and hence cause the body to heat up.
This phenomenon is, however, not only valid for absorption but also for the release of heat through radiation: the brighter and smoother the surface of an object, the less is its ability to radiate and consequently the longer it retains its heat. On the other hand, with a dark, rough surface, an object can cool down very rapidly as a result of radiation.
The dullest black and least brightly reflecting surfaces show the strongest response to the various phenomena of thermal emission and absorption. This fact would make it possible to control the temperature of objects in empty space in a simple fashion and to a large degree.
Figure 71. Heating of an object in empty space by means of solar radiation by appropriately selecting its surface finish.
Key: 1. Object; 2. Solar radiation; 3. Highly reflecting surface (impeding cooling by emission); 4. Dull black (causing heating by absorption).
If the temperature of an object is to be raised in space, then, as discussed above, its side facing the sun will be made dull black and the shadow side brightly reflecting (Figure 71); or the shadow side is protected against outer space by means of a mirror (Figure 72). If a concave mirror is used for this purpose, which directs the solar rays in an appropriate concentration onto the object, then the object's temperature could be increased significantly (Figure 73).
Figure 72. Heating of a body by protecting its shadow side against empty space by means of a mirror.
Key: 1. Mirror; 2. Polished side; 3. Object; 4. Solar radiation
Figure 73. Intensive heating of an object by concentrating the rays of the sun on the object by means of a concave mirror.
Key: 1. Concave mirror; 2. Object; 3. Solar radiation.
Figure 74. Cooling an object down in empty space by appropriately selecting its surface finish.
Key: 1. Object; 2. Solar radiation; 3. Dull black (promoting cooling through emission); 4. Highly reflective surface (impeding heating by radiation).
If, on the other hand, an object is to be cooled down in outer space, then its side facing the sun must be made reflective and its shadow side dull black (Figure 74); or it must be protected against the sun by means of a mirror (Figure 75). The object will lose more and more of its heat into space as a result of radiation because the heat can no longer be constantly replaced by conduction from the environment, as happens on Earth as a result of contact with the surrounding air, while replenishing its heat through incident radiation would be decreased to a minimum as a result of the indicated screening. In this manner, it should be possible to cool down an object to nearly absolute zero (273° Celsius). This temperature could not be reached completely, however, because a certain amount of heat is radiated by fixed stars to the object on the shadow side; also, the mirrors could not completely protect against the sun.
Figure 75. Cooling an object by protecting it against solar radiation by means of a mirror.
Key: 1. Object; 2. Solar radiation; 3. Mirror; 4. Reflective side.
By using the described radiation phenomena, it would be possible on the space station not only to provide continually the normal heat necessary for life, but also to generate extremely high and low temperatures, and consequently also very significant temperature gradients.