How is the temperature of outer space measured?

You can’t measure the temperature of a vacuum the way you measure the temperature of a physical object as there is nothing to measure. What you can do is measure the emitted radiation. All objects emit radiation according to their temperature and by measuring the background radiation emitted from space you get a temperature of around 2.72K.

If you were floating in space you would lose heat through radiation and would cool down to the temperature of space unless there was a nearby sun to warm you up.

Sending this to Rocketguy who knows a definitive answer since he designs satellites.

One thing I haven’t seen mentioned is the effects of vacuum on the boiling point of bodily fluids. Turning liquid into gas involves heat transfer; you wouldn’t be frozen so much as freeze-dried.

This is Greek to me, but you might be able to understand.

@bkcunningham Those Straight Dope columns are always entertaining & informative. I think Cecil just likes to throw around formulas, though. All that talk of Planck’s law and Wien’s displacement etc. are just details of how to calculate temperature from an observed distribution of wavelengths, which in this case are in the microwave band.

Heat is, very basically, molecules in motion. Space, by definition has fewer (much fewer) than what we are used to here in our homely little atmosphere. It is a vacuum, relative to what we safely enjoy here on Earth. Due to this extreme lack of molecules to measure the movement of, we do what @flutherother talks about and we measure radiation.

We can’t use a thermometer like we use in our homes because they use the physical properties of earth that create ‘thermal equilibrium’ and you don’t get that in a vacuum. We also have to use the Kelvin Scale, which starts from a theoretical position of no molecular motion, known as absolute 0.

What they can take the temperature of, when designing craft and satelites, is the temperature of the craft or satelite and the radiation effects on its temperature it will experience in orbit. This is what they mostly care about and consider. The changes in temperatures it will need to withstand, for example. When the craft is not shaded by the planet or moon, it will experience a great deal of direct solar radiation and even indirect radiation of light coming from the planet or moon it is orbiting. This is measureable on and in the spacecraft because of the molecules. They design systems and materials that disappate the radiation so it doesn’t become ‘heat’.

Does that make sense? I’m really tired and feel like I am rambling a bit.

Pretty easy one: with a rectal thermometer inserted in Uranus.

@cazzie It’s true that temperature is a measure of average kinetic energy of particles, so without particles that definition is meaningless. You say, …the physical properties of earth that create ‘thermal equilibrium’ ... you don’t get that in a vacuum. In thermal equilibrium the energy density is the same everywhere. This includes both particles (characterized by a temperature) and light / photon energy (characterized by a wavelength). Photographers adjust the “color temperature” of lights.

Given time to reach equilibrium, particles, if any, will gain or lose energy to equilibrate with the blackbody temperature based on electromagnetic radiation alone. In the human realm we often don’t achieve (or want) thermal equilibrium – you still have to wear a coat in winter.

Think of a glass artist’s oven. It glows a uniform orange in thermal equilibrium at a few thousand degrees, even when empty. Anything inside (glass, air, etc) is also at this temperature and emitting light of that color. Macroscopic objects become invisible because everything has the same uniform glow. If we could see in infrared, people would become invisible in the same way just by heating the room to body temperature!

Anyway, about a hundred thousand years after the big bang the universe cooled to just a few thousand degrees. Ionized plasma could now form neutral atoms – at a temperature of 4000K. For the first time ever, those photons could travel unimpeded through neutral space to penetrate the entire universe. That’s the “afterglow of the big bang.”

How did it go from 4000 degrees to 2.7? Red shift (doppler effect). If the universe didn’t expand then all of space would still be at 4000! When you stretch light waves you increase the wavelength which lowers the energy. Today space is cold.

@gasman But not uniformly; read @bkcunningham‘s link.

@rojo‘s answer brought out the 6 year old in me. ROFL

I was trying to explain how you can’t use a regular thermometer in space, @gasman, like the ones here on earth. I can see how I didn’t explain that clearly.

@poisonedantidote, @bkcunningham found the 1000 word explanation for you.

My 100 word explanation would be to consider space to be an area with almost no heat energy (temperature near absolute zero). Since most things we can touch contain some heat energy (temperature > absolute zero), when these things go into space their heat energy will escape to areas with less energy. They will become colder with time if floating in space.

Things on Earth actually lose heat to space all the time, but air and clouds will block some of that heat flow. Cloudless nights allow more heat to escape than cloudy nights.

Did you know the coldest place in the (known) universe is here on earth? Deep space is about 2.7 Kelvin but here on earth we have managed to get liquid helium down to a few billionths of a degree above absolute zero (0K). At that temperature when you shine light through it the speed of light drops to almost nothing.

Random I know but I felt like sharing