Ask Us

Space Physics:
Heat, Temperature, and the Electromagnetic Spectrum

Temperature of the Universe

What is the approximate temperature of the Universe, and how can it be calculated?

The entire Universe is filled with the remnants of the Big Bang, in the form of photons (electromagnetic packets). They have cooled down to about 2.7 Kelvin or 2.7 degrees above absolute zero (-270.7 degrees Centigrade). So this is the temperature of space. It can be calculated from the expansion of the Universe, and it has been measured.

You can learn more about the COBE mission that measured this on the TopHat web site.

Drs. Eric Christian and Louis Barbier

Temperature of the "Void"

Is it correct that objects in deep space (not heated by an external source, such as the Sun) have a temperature near absolute zero - but the void itself has NO temperature?

Objects in the void will eventually come to the equilibrium temperature of the cosmic microwave background, which is 2.7 Kelvin (2.7 degrees above absolute zero). The cosmic background is a sea of photons (light) that are the remnants of the Big Bang, which has cooled down to 2.7 Kelvin over 15-20 billion years. The comment that the void itself has no temperature comes from the fact that temperature is usually defined with the random motions of matter, and if there is no matter, there is no temperature. But I (and many others) equate the cosmic background with the 'void' having an effective temperature of 2.7 Kelvin, even if there is no matter.

Dr. Eric Christian

Does Heat Travel Through a Vacuum?

Does heat travel through a vacuum, and if so how? If not, how does the Sun heat the Earth?

Heat travels through a vacuum by infrared radiation (light with a longer wavelength than the human eye can see). The Sun (and anything warm) is constantly emitting infrared, and the Earth absorbs it and turns the energy into atomic and molecular motion, or heat.

Dr. Eric Christian

Heat in Space

When I think of "heat," I think of the energy of atoms bouncing against each other or against a wall. The gas in space cannot be dense, so the chance of one atom bouncing against another seems to be zero, and there is no wall. How can there be heat?

Your question is a very interesting one, as it reveals a rather common misunderstanding of the meaning of "hot." First, your understanding of heat is indeed quite correct. Heat is in fact the average kinetic energy (or the energy of motion) of the individual atoms (or molecules) in a gas. The faster the atoms are on average, the higher their kinetic energy, and the hotter the gas is.

Now, there is an important distinction to be made between "a gas is hot" and "a gas contains a lot of heat." Any gas, whose atoms bounce around at high speed, is considered "hot." However, in order to contain lots of heat (i.e. lots of heat energy) the gas ALSO has to be dense, i.e. contain lots of atoms in each cm³. In this sense a gas in space, such as that of the solar corona with its 1-2 million degrees Kelvin (or 2-4 million Fahrenheit) or the solar wind, which stems from the hot solar corona, is quite "hot," when it comes into contact with a spacecraft it is not capable of transferring lots of heat energy, because it is such a thin gas. While the air we breathe contains approx. 4 * 10¹⁹ molecules/cm³, the solar wind only contains a few (typically 1-30) protons (the naked nuclei of H atoms) and a few electrons per cm³.

See also my answer to a related question about the heat of the Sun's corona.

Dr. Eberhard Moebius
(October 2004)


The Cold of Space

NASA frequently refers to "the cold of space" and the extremes of temperature from the sunny side of the shuttle to the shaded side. If I remember my college physics and astronomy correctly, space is a vacuum. Doesn't a true vacuum have no temperature? It would seem that the only problem that the astronauts should have is getting rid of heat. The only form of heat dissipation would be radiational and that would not be enough to cool an astronaut in a space suit -- they must require some form of air conditioning. If I am incorrect, then how does a vacuum bottle keep things hot or cold?

You remember your college physics correctly. Space is a vacuum, and heat can only be exchanged through radiation. However, that is a quite powerful means of exchanging heat. Have you ever stood in front of a campfire on a very cold winter night? While facing the fire you may feel roasted in your face, while your back feels frigid. The fire radiates heat at you, and your back radiates heat into the cold night. Of course, the cold air around you plays a role, but if there is no wind, the major heat exchange is radiation.

In space this is turned to the extreme. Without any star or planet nearby the temperature of space (as defined by radiation) is 3 K (-270 centigrade), the temperature of the ubiquitous background radiation from the Big Bang, i.e. extremely cold indeed. A spacecraft is roasted on the side that faces the sun and very effectively cooled on the opposite side.

The amount of heat that a spacecraft radiates into space and receives from the Sun can be controlled by the makeup of its surface. And this is the second secret of the vacuum bottle (or thermos): while the vacuum suppresses heat exchanges by conduction and air convection, exchange by radiation is suppressed by the shiny metallic coating of the bottle. This shiny coating reflects the heat radiation like a mirror and keeps it either inside the bottle (if the content is hot) or outside (if the content is cold).

Dr. Eberhard Moebius
(March 2003)


Why Do Temperatures Vary So Much?

Why do temperatures vary so much in the absence of air?

Temperature is a measurement of the amount of thermal energy or heat of an object. On the microscopic level, heat is a measure of the speed of the atoms and molecules in the object. If an object is surrounded by something (say air or water) at the same temperature then it will remain at that temperature. However, if the object is surrounded by something at a lower temperature, it will transfer some of its energy to its cooler surroundings and, unless that energy is replaced, it will grow colder. And, if an object is surrounded by something at a higher temperature, some of the energy from its surrounds will be transferred to it and, unless that energy is replaced, its surroundings will grow colder.

There are three ways this heat can be transferred into or out of an object. These are called conduction, convection, and radiation. Conduction is thermal energy (heat) moving within a body. (Touch a match to one end of a metal bar and the far end, as well as the near end, gets hotter. In this case no material within the body actually moves.) Convection is heat transport by moving material from one place to another. (Heating a pan of water causes hot water, which is less dense than cold water, to rise to the top and the cooler water at the top to fall to the bottom. This transfers heat from the bottom throughout the volume of water.) Radiation is heat energy carried by electromagnetic waves. (This is how the heat from the Sun is transferred to other objects in the solar system, including the Earth.)

Objects in space (that is, in the absence of air) are heated by radiation on their Sun-facing side and then, because there is nothing (like air, which acts as a "blanket") to reflect and reradiate the heat back toward the side facing away from the Sun, most of that heat escapes into space. This also happens to the Earth, which is heated by the Sun during the day and radiates some of that heat back into space during the night. However, the atmosphere acts like a blanket and keeps much of the heat from escaping during the night. Clouds are an even better "blanket" than air because clouds (the kind made of droplets of liquid water) absorb more of the heat radiation escaping from the surface into space than clear air.

So, (to finally get to the answer!) temperatures vary so much in the absence of air because there is nothing to reflect, absorb, and scatter the entering heat and nothing to hold in the heat on the "night" side. This is why the day/night temperature of the Earth in the northern US during a typical June is 21°C / 11°C (70°F / 52°F), whereas that on the Moon (located at the same distance from the Sun as the Earth but lacking an atmosphere) is 115°C / -180°C (240°F / -290°F).

Dr. Ed Tedesco
(December 2004)


How Does a Spacecraft Dissipate Heat?

If vacuum is used in thermos bottles to keep heat or cold from leaving, how can a spacecraft get rid of any excess heat or cold in the vacuum of space?

There are three ways of transferring heat: convective, diffusive, and radiative.

• Convection transfers heat by moving matter, usually a gas, around. A fan blowing on you is convective cooling.
• Diffusion is the transfer of heat through contact, like putting your hand on an ice cube.
• Radiative transfer uses photons (light or infrared photons) to transfer heat.

A thermos bottle gets rid of convective heating or cooling with a vacuum, and only a small amount of diffusion happens through the top and the glass wall. The silvering of the glass helps limit radiative heating or cooling. But the thermal radiation is ALWAYS there, and that is what a spacecraft uses. To get rid of heat, you can point thermal radiators at the dark sky, and to warm up you can point at the Sun or Earth. The Sun warms the Earth through radiation, not convection or diffusion.

Dr. Eric Christian

Gamma Rays

Can you recommend one or more good basic gamma ray web sites?

Here are some web sites I know of that will provide you with information on gamma rays:

• Imagine the Universe! has information about what gamma rays are.
• The Astrophysics Data Facility at NASA GSFC has a picture of the Milky Way in gamma rays.

I hope you find these sites helpful.

Laura McDonald -- "Guest Answerer"
Energetic Gamma Ray Experiment Telescope (EGRET)
Compton Gamma Ray Observatory (CGRO)