Feeling of Heat in the Thermosphere

Question:

R.J.Montemayor, Grade: 7-9
Groveton, Texas
United States
Area: Earth Sciences

Molecules are far apart in the thermosphere, so a thermometer doesn't register the 2000 degrees Celsius plus that it is. It registers below 0 degrees Celsius. Would something actually burn up in this layer? Explain why or why not.

Answer:

No, a person who was sitting in the thermosphere would not feel the heat of the atmosphere at all; like the thermometer, he would probably feel cold.

Willy Ley, one of the great space pioneers, explained it this way for me long long ago in his classic book Rockets, Missiles, and Space Travel:

"You wouldn't expect a person to be burned to death in a cold barn by the admittedly very hot filament of the light bulb. The temperature is indeed very high, but the quantity is utterly insufficient."

The temperature of a body or a person in space depends on the balance of heat energy received, from many sources, against energy lost by radiation of infrared ("heat radiation") to the environment. The energy received would include (in approximate order of importance):

• Radiation from the Sun (including both visible light and also infrared and ultraviolet rays, and a few X rays).

• Infrared radiation from the ground and deeper parts of the atmosphere.

• Sunlight (during the day) reflected from the ground or clouds.

• Any energy received from the spaceship or whatever the body might be connected to.

• A very tiny bit of far infrared radiation received from space due to the cosmic microwave background at T=2.73 Celsius degrees above Absolute Zero; plus a bit from the Moon and planets.

• Heat conducted from the atmosphere -- the very hot gas of the thermosphere.

  (Notice that people or spacecraft in the thermosphere are almost always moving very fast -- in reality nobody just sits there! Since "this gas is hot" means "the molecules of this gas are moving helter-skelter very fast", the result is that the molecules crashing into the front of the spacecraft will seem very much hotter than their actual temperature. Then everything will depend on just how fast and just how high the spacecraft is. This, of course, is why meteors and spacecraft burn up in the atmosphere.)

Each of the above sources has both a temperature and a quantity associated with it. As energy flows into the body from those various sources, its temperature will naturally tend to rise. However, as it becomes warmer, it will radiate more and more energy (as infrared radiation) back into its surroundings. Eventually the energy received and the energy radiated will come into balance, and then the temperature of the body will hold steady.

For something near the Earth in space, the main factors determining the temperature in practice are how much energy it receives from the Sun and Earth, and its color (light or dark) in the infrared, which determines how easily it can lose energy by radiation to the surroundings. The result will usually be in the range -100° to +100° C; which is a bit too large to leave to chance! Thus the thermal engineers, who are the people who figure all this out in detail, are among the most important experts in designing spacecraft and space missions.