Is Space Cold or Hot?

Such a simple question, and yet not the simplest answer. In order to better understand this question, we must first understand the difference between heat and temperature. That’s right…they are not the same thing! But why?

It may seem counter-intuitive that temperature and heat are different things. After all, if we heat something up, its temperature increases, right? Things put into a flame feel hotter…things in a refrigerator get cold…so how could these be separate? To tease out the difference, we need to understand the definitions of temperature and heat, and how these definitions turn into physical meaning.

First up: heat. Heat is a measure of energy (in energy units) in a substance: the greater the total energy, the greater the heat. Remember that there are different types of energy, such as kinetic energy (energy of motion), internal energy (e.g. energy in molecular bonds, energy of the mass making up matter), and potential energy (the energy available to do work).

Next: temperature. Temperature is a number that is proportional to the average kinetic energy of the molecules in a substance. When molecules (or even individual particles) have a high temperature, then they are moving faster than at a lower temperature. This is why water at high temperatures cannot solidify—the particles have too much energy to get locked into a crystal state.

So here’s the crux of the issue: when particles have heat transferred to them, their temperature increases. So lets take a quick look at how heat can be transferred between particles, atoms, and molecules. The first way is through radiation. Radiation is essentially a source of energy giving off its heat in the form of light. Think light bulbs, stars, and even humans–we radiate off some of our heat in the infrared, which is why you can see us with “night goggles” (an infrared radiation detector). The hotter something is, the more it radiates. The second method is called convection. This method of heat transfer involves patterns of rising and falling gasses of different temperatures, and is very important in the transfer of heat near the surface of the sun! (And in your house—we all remember learning about hot air rising and cool air falling, right?) Inside the sun’s upper layers, the hot plasma rises up to the surface, releasing its energy and cooling down. Then, it sinks to the bottom and where it can absorb more energy and heat up again (see the picture below). For those of you who like computers, convection is what we use to cool down our computer chips when they heat up–the fan blows cool air over the chips, and the air absorbs some of the heat from the chips.

The most important heat transfer mechanism that we need to consider for this question is the one we experience most often on Earth, the mechanism of conduction. This is essentially heat transfer through particle collisions. Remember, we said that when a substance’s temperature is high, the particles that make it up have a lot of kinetic energy, so they bounce around a lot. It makes sense then that as those particles collide with other particles, some of the heat gets transferred from the hotter particles to the colder ones. Think about when you stick the end of a metal spoon in a fire. What happens? At first, the tip of the spoon heats up (energy from the fire is transferred to the particles in the metal spoon’s tip). Then, over time, even though you are holding the spoon at the other end, you burn your hand. That’s because the particles in the hot tip of the spoon smack into the colder particles adjacent to them, and the heat begins to move along the spoon to the tip–eventually, the other end of the spoon gets hot, and you burn your hand (and feel really stupid for having done that).

"Feelin' hot, hot, hot!"

On Earth, conduction plays a huge role in energy transfer because we live in a high density environment. Air is everywhere, so if you start heating up air in a room, it spreads out quickly–heat is easily transferred between air molecules. But what about in space?

If you’ve followed me so far, you are now ready to answer the question posed in the title of this post. In the environment of our solar system, particles are moving very fast, that is to say, they have a very high temperature! Most of these particles are from the sun’s solar wind—the sun doesn’t just send out light energy, but also a constant stream of energetic, charged particles. These particles move at close to 900 miles per hour! That’s freakin’ fast…and corresponds to a temperature of hundred thousands to millions of degrees! Soak that in a moment.

But what about heat? These particles may have a very, very high temperature, but there are very few of them. Space is almost a complete vacuum, with very few particles. If you were to sample a space of 1 cubic centimeter, you’d find about 5 particles in it. So heat isn’t able to transfer effectively. Conduction is essentially impossible—you can only transfer heat through radiation.

This has some really interesting consequences. If you cannot conduct heat effectively, temperatures stay roughly the same for long periods of time–so things that are cold stay cold, and things that are hot stay hot. So when you are directly in the sun, you will actually feel pretty hot–temperatures on the dayside of Earth outside the International Space Station can get into the 100s of degrees Fahrenheit! But then, on the night side, when not in sunlight, it gets really cold, into the negative hundreds of degrees. And once you are at a certain temperature, you aren’t able to effectively transfer it away, so you’ll stay that way–so in space, if you had a hot cup of coffee, it would stay hot for longer than it would on Earth! Yeah…space is the ultimate thermos.

What happens if you are not near the sun at all, like the temperature between galaxies? When not in the presence of a heat source in space, you will only feel the Cosmic Background Radiation on you, which is a temperature of 3 degrees Kelvin (-455 degrees Fahrenheit)! So…you’ll freeze (though that’d be only one of the problems you’d face). That concludes our lesson on heat and temperature! Now go out and enjoy Groundhog Day with a little Ryan Gosling:

I feel my temperature rising!

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