|MadSci Network: Earth Sciences|
The very concept of temperature is not a fundamental one like mass or charge. It is actually a measure of the average kinetic energy of all of the particles in a system. It does not have very much meaning except when a system is itself at thermal equilibrium. And its value depends very much on the nature of the thermal contact between a system and its surroundings. Interplanetary space is a fairly good vacuum. There is little opportunity for heat to move to or from an object by conduction or convection. Small amounts of energy will be transferred in occasional encounters with the molecules of interplantetary space. Most heat transfer to or from a body in interplanetary space will be through radiation. I presume that you are asking the following question: suppose that we were to take an object to the edge of the earth's atmosphere, where the background molecule concentration was similar to that in interplanetary space. We leave it there for a long time until it achieves some sort of thermal equilibrium with its surroundings. What temperature will we find it at? There is a need to be even more precise in the first part of your question. Something in direct sunlight has a sunny side and a shady side; at most one half of it is in direct sunlight. So I will presume that your object is made of metal (so that heat will be rapidly conducted through the whole object), or rotating rapidly with an axis perpendicular to the plane of the earth's orbit, or both. Finally, if we are looking at radiation transfer into and out from an object, it is important how much of the radiation is absorbed, and how much is simply reflected away from the surface. The proportion of incoming sunlight that is reflected directly back into space by an object is known as its albedo. Initially, we will assume that your object is a perfectly black body, that is, its albedo is 0. In this case, the required datum can be found in the CRC Handbook of Chemistry & Physics, in a table entitled 'Physical Data for the planets, their satellites, and some asteroids' in a column headed 'Average Temperature (K)' and sub-headed 'equilibrium'. In my volume (56th Ed) it is on page F-176. The equilibrium temperature of a black-body in direct sunlight in interplanetary space near the earth would be 394 K = 121 deg C. If the body is not black, a good approximation can be obtained simply by multiplying (1 - albedo) by the absolute temperature. For the earth itself, the albedo is about 0.36. 36% of total incident sunlight is reflected back into space, which means that 64% is absorbed by the Earth/atmosphere system. The equilibrium temperature for a body of this brightness would be 0.64 times 394 K = 252 K = -21 deg C. The equilibrium temperature for a body as bright as the earth in direct sunlight in interplanetary space near the earth would be 252 K = -21 deg C. The earth itself is some 35 degrees warmer than this because of the natural greenhouse effect due to the water vapour and carbon dioxide in its atmosphere. The second part of the original question is much more difficult to answer in any meaningful way at all. An answer I can give is 'definitely less than 100 K = -173 deg C, and definitely more that 2 K = -271 deg C'. If direct radiation from the sun is not part of the energy input, then what is? To what extent does the object causing the 'shading' block access of energetic solar particles? Is the object itself exposed to bright 'earthlight' -- reflected sunlight from the earth? It is 'just above the earth', but how much of the infrared emission of the earth's own radiation it will receive depends exactly on just how far above the earth. The figure of 100 K is the temperature of the night side of the moon. Basically that is 'an object just above the earth in the shade'. But this temperature will be a fairly drastic over-estimate, because much of it will be due to a residual warmth from when the local surface was on the sunlit side of the moon a fortnight previously. The temperature of 2 K is the temperature of the background microwave radiation that is believed to be the echo of the 'big bang'. As the whole universe is bathed in this radiation, it probably represents the lowest equilibrium temperature that any object can achieve in the universe.
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