What Would a Thermometer Say the Temperature of Space Is?

Introduction

I just took my temperature and it read as 97.4 °F. A normal human body temperature. But the thermometer outside says it’s 6 °F. It also says it’s -14 °C. But being in the physical sciences discipline, I may want to say it’s really 259 K.

But what would that thermometer read in space? If the astronauts when they went to the moon took out a household thermometer, would it read around +200 °F, the reported surface temperature at noon? Or would it read millions of degrees because that’s the temperature of the particles from the sun that are streaming past it? Or, would it read nothing at all?

The Concept of a Temperature and Reliance on Material to Measure

The English word “temperature” originates from the Latin temperatura, which originally denoted a state of being tempered or mixed, later becoming synonymous with “temperament.” In the 1600s, it was co-opted to its present-day usage, which has a definition of, “the degree or intensity of heat present in a substance or object,” according to the built-in dictionary on my Mac. So in other words, “temperature” is really a measurement of “heat” … and we must define “heat.”

In physics, the concept of “heat” comes from objects that are in motion and, in that sense, it can be used synonymously with “kinetic energy” (the energy of motion). If all molecular/atomic motion in a substance were to stop, then we say that it has zero heat, consequently zero temperature, and we would say it has a measured temperature (if it could be measured) of 0 K.

“K” stands for “Kelvins,” and it is unique among temperature scales in that there is no “degrees Kelvin,” just “Kelvins.” Intervals of 1 K are equivalent to 1 °C, so the Kelvin scale can be thought of the Centigrade (Celsius) scale with a different zero-point. 0 K = -273.16 °C = -459.7 °F = 0 °R (where “°R” is “degrees Rankin, the same scale as Fahrenheit except with a different zero-point).

Okay, so now that we have a physical definition of temperature, what does it mean when we measure temperature? To measure temperature in a normal, every-day way with a thermometer is to allow the thermometer to interact with the substance it is measuring until the same amount of energy / heat / molecular motion is reached by the thermometer. Then we assume that the thermometer has a read-out which tells us what that corresponds to on a scale such as Celsius or Fahrenheit.

A key to this temperature measurement, however, is in the phrase, “allow the thermometer to interact with the substance it is measuring.” In space, this is not so easy.

How Heat Is Transferred

I covered this in my post “Apollo Moon Hoax – Huge, Deadly Temperature Variation Claims”. However, to review, there are three ways to transfer heat: Radiation, Conduction, and Convection.

1. Radiation: Radiation is the least efficient process of transferring heat. It involves exactly what it sounds like – radiation, or light-based energy (photons). The photon is emitted from the heat source and is absorbed by the target. The act of absorbing the photon – a packet of energy – adds to the energy of the target material, thus heating it up.

   The Sun heats all objects in the solar system mainly through radiative heat transfer. This is also the same mechanism behind “heat lamps” – those hot red lights that are oh-so-common in cafeterias, keeping the french fries or the pizza warm.

   There’s another aspect to this that does not play a role in the other two heat transfer processes: Some surfaces will absorb heat faster than others. This is because objects that are whiter will absorb less radiation because they reflect more. Objects that are blacker will absorb more radiation because they reflect less. Astronomers call this “albedo.” You may have noticed this effect if you’re outside in the summer and wear a white shirt vs. a black shirt – you’ll heat up much more quickly in black.

2. Conduction: Conduction is the process where heat is transferred by one object physically touching another object. For example, when you place a pot of water on the stove to boil, the heating element of the stove physically touches the pot, heating it up, and the pot physically touches the water, heating that up.
3. Convection: Convection is the most efficient process of heat transfer. It involves the physical mixing of material of two different temperatures, which distributes the heat. An everyday example of this is adding ice to a glass of water and then stirring it around. This stirring physically moves the ice and water to better distribute the heat than if the ice just sat there (conduction).

   Another good example is a pot of thick stew or chili on the stove. I learned this lesson the hard way – while soup convects quite easily, chili only conducts. In other words, in most soups, you generally get a good boil going and the liquid circulates throughout the pot, carrying and distributing the heat very well. Thicker foods like chili, however, do not convect; the heat conducts up through the pot to the food on the bottom, and then it just stays there. The bottom will continue to absorb heat, but because the food is so thick, these warmer parts of the food don’t move anywhere, they just sit there, slowly conducting heat away at a slower pace than the pot is conducting heat to it. This results in burnt chili on the bottom and barely warm chili on top.

So What Does a Thermometer Measure?

A conventional thermometer reaches an equilibrium when the rate of heat radiated away is equal to the rate of heat absorbed through conduction. In all, every-day situations, a thermometer measures “temperature.” This is because the process of radiating heat away is much less efficient than conduction when you are surrounded by a material as dense as, say, the human tongue, a pot of water, or even air.

But now, let’s take that thermometer and put it in interplanetary space. In our solar system, the space between planets (hence, “interplanetary”) is not very dense. It’s less dense than the best vacuum that we can create on Earth. But, it does have material in it. There’s dust and gas, and there are streaming charged particles from the sun that we call the “solar wind.” So what would a thermometer measure?

The dust and gas is fairly cold unless the stray molecule gets hit by a cosmic ray or particle from the solar wind, which will heat it up (but then slowly radiate that heat away). The space between particles and molecules isn’t baryonic matter, and hence even if there’s something there, it can’t interact with the thermometer. The solar wind is very hot – each particle the same temperature as the chromosphere of the sun, several millions of degrees (or Kelvins – they’re fairly interchangeable when you’re talking about the general “millions”).

However, the time that it would take the material – the dust, gas, and solar wind – to conduct heat to the thermometer is longer than the time it takes the thermometer to radiate heat away. And so the thermometer would slowly decrease in the temperature that it records until it reaches 2.73 K, the temperature of the Cosmic Microwave Background Radiation. There are enough photons left-over from that to keep the thermometer at least at 2.73 K, and maybe it will read a slightly warmer temperature if it gets hit by enough solar wind particles.

Then How Do Astronomers Say that a Dust Cloud Is Millions of Degrees?

Since conventional thermometers, for all intents and purposes, don’t work in space, astronomers have to use other means to estimate the temperature of objects. They do this by examining the light emitted, and the wavelength at which the most intensity of light is emitted will uniquely determine the temperature: Redder objects are cooler, bluer objects are hotter. If you’re interested in reading up on this, check out Wikipedia’s pages on Blackbody Radiation, the Planck Function, and Wein’s Law. I’m not going to go into the actual physics and mechanics on this post because that is not the purpose of this post.

Final Thoughts

You may ask, as on a few of my other posts, why I bother to talk about something that seems to be fairly insignificant, or splitting hairs. Well, as usual, my reply is that it’s because this is a commonly held misconception, that if you put a thermometer in space, it will tell you how cold the material around you is. The misconception stems from not understanding how heat is transferred between objects, and this itself leads to a host of other misconceptions, like what would happen to a human if they were put into space without a spacesuit, or why the Apollo astronauts were able to easily survive in the cold and then hot temperatures they encountered on the moon.

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