How cold is Space?

Do you think space, or rather the vacuum, is cold? During my survey on the web during the past few months, it clearly showed that most people would answer yes to that question.

Fact is that only matter can have a temperature, which is the speed of its molecules. The faster the molecules in a material move, the warmer that material is. Per definition, an absolute vacuum is a void, nothingness and therefore it cannot have any temperature. In as far as the space of the Universe is an absolute vacuum, that space cannot have any temperature and it is thus not "cold".

However, an absolute vacuum does not really exist; there are molecules whirling around everywhere in space. Actually, some recent  theories say that all these molecules whirling around, constitute more mass than all the visible galaxies contain together - it's called "dark matter". Even so, the density of these molecules is so incredibly low, that in practical terms, as far as space technology is concerned, interplanetary space behaves as an absolute vacuum. Even the Moon, that actually does have an atmosphere in the sense that the density of its gas molecules is considerably higher than in "free" space,  can yet be seen as an absolute vacuum environment in practical terms for human activity there. This means that the molecules that are there, do not have a measurable contact (convection) heat exchange effect with other materials around and thus a Moon vehicle or base on the Moon can neither be cooled, nor heated by these molecules. The same is valid for space vehicles traveling around in the Solar System; they also have no measurable heat exchange with the molecules moving in the vacuum around.

Even on Mars, that has a well defined atmosphere, such heat exchange effects would not have much significance for human activity there, though it would be noticeable nonetheless. Surely, the air temperature on Mars can locally come up to plus 30 degr. C, but that doesn't mean you would "feel" it, the same as on Earth, because the Mars air is so much thinner. The atmospheric pressure on Mars is just 6 mbar, compared to Earth's atmospheric pressure of 1000 mbar.  No industrial "vacuum" pump on Earth could reach such a low pressure and it is yet called a vacuum pump. Hence, in technical terms, also Mars could be seen as a vacuum environment for astronauts, just not an absolute one, as it is on the Moon.

This all means that space, the vacuum, is a perfect temperature insulator for convection heat. The only heat exchange that can be done between bodies in the vacuum is through radiation, because the vacuum lets electromagnetic energy pass through. Very fortunate, so we can get light and warmth from the Sun, all being electromagnetic radiation. The frequency of this radiation is a measure for how much energy it transmits. The higher the frequency, the more powerful the radiation is and usually penetrates deeper into materials. X-rays have a very high frequency and are therefore powerful enough to penetrate our bodies, which for example is used in medical applications. Gamma rays are even more powerful and are generated by decaying atoms - nuclear radiation. Heat radiation is called infra-red, because the color red is the lower frequency limit of what our eyes can see (violet the upper).

Infra-red has a too low frequency for our eyes to see and we feel it as heat instead. Its frequency relates to the temperature of the emitting body, the higher the frequency, the warmer that body is. This causes the so called green-house effect, because certain materials are more transparent for higher than for lower frequencies. The surface temperature of the Sun is 6000 Kelvin and the according frequency can penetrate the Earth's atmosphere. As it hits the ground, most of it gets absorbed and warms up the ground material - ever noticed how hot beach sand can be?

However, the thus generated temperatures are much lower (fortunately) than that of the Sun and the Earth's atmosphere is not transparent for the according lower frequencies and so it warms up, partially by absorbing the energy that the warm ground radiates off and partially through convection heat exchange between air and ground material molecules and part by what it absorbs itself directly.  The same principle is valid for glass and that's why your car gets so hot inside, when it is parked in the sun. Likewise the temperature in green houses rise above surrounding air temperatures, as is the purpose of those.

The warm atmosphere of the Earth radiates off heat into space at a far lower frequency than what it received from the Sun and this heat disperses into space, without "warming" it up. Space, the vacuum, cannot have a temperature and so the heat energy that the Earth's atmosphere radiates off, disperses into larger and larger volumes of "nothingness". The Earth is thus not cooled by any "cold" space, because that would require convection, which the vacuum cannot provide.

Likewise, the distant planets are very cold, because they receive very little energy from the Sun, not because they are surrounded by "cold" space. Any object in "dark" space, not receiving any heat, nor generating any itself, will become extremely cold, as it radiates off whatever little it still has. How cold, we'll see at the end of this article.

The average temperature of the Earth is determined by a balance between received and given off heat energy. The atmosphere's temperature stabilizes there where both amounts are the same. Hence, the Earth gives off as much energy as it receives from the Sun; nothing is "consumed", or "used" as many erroneously think. The same is valid for a green-house and you car parked in the sun; the inner temperature stabilizes at a value where energy balance is reached.

Of course, not all the solar energy that hits the Earth is absorbed by it. Much of it is reflected back into space. From the rest, the atmosphere absorbs a part itself and lets through a part to reach the surface. As long as the properties of the atmosphere do not change, the Earth's global temperature will not change, but if we bring about noticeable changes with our emissions of whatever gases, anything can happen. The Earth can become cooler or warmer. Today the talk is about global warming, but there are scientists who argue for a risk of global cooling also. In the end, nobody knows for sure, because the heat household of the atmosphere is a very complicated system, with many unknown parameters.

However, if it ever would happen that we release so much heat from fuels, that it becomes a noticeable part of the Earth's total energy household, we would indeed warm up Earth by it. I don't think that ever will happen, it's just a theoretical exercise, but we do cause heat-pollution locally, warming up waters around large power plants that are cooled by them and it affects the biological systems there.   

On Earth, cars, green-houses and whatever other structures, are heavily cooled by the surrounding air, especially if there is wind blowing around them. Not so on the Moon for example. If you see science-fiction designs of huge glass cupolas on the Moon and  space-crafts with large glass windows all around, you are basically looking at ovens. If exposed to sunlight, they would self-destruct by overheat, unless practically all of the solar heat is reflected (not absorbed !) by such glass, or whatever transparent material. Even then, such habitats are additionally heated by the body heat of people in there and by the power supplies to run technical systems (all energy decays to heat at ambient temperature - Second Law of Thermo). All that heat must be radiated off also.

How cold can an object in "dark" space become? Many say 3 Kelvin, which is the "temperature" of the cosmic background radiation (actually 2.7 K). This radiation is assumed to be a remnant of the Big-Bang and its frequency corresponds with 3 Kelvin. Many think erroneously that this is the temperature of space, but that is of course not true. An object in deep, deep dark space can become colder than 3 Kelvin, but the Second Law of Thermo says it can never become 0 Kelvin, because 0 Kelvin is not a temperature - it is the absence of it,  "nothingness", a void. Matter as we know it, cannot exist at 0 Kelvin,

However, this is not agreed upon by all scientist. Look for the "Third Law of Thermodynamics" in a search engine and you will find it - I personally do not agree with that "law".

Btw, do you want to raise your voice over Climate Change, Global Warming? Read about Spaceship Earth first!