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Fahrenheit 4,118

Now open the door to a hot kitchen oven: The cook will be blasted by air molecules flying out of the oven at tremendous speed. These air molecules have been violently excited by the confines of the hot oven. As the molecules hit the cook in the face, they impart lots of energy-heat. But heat exists only where there's a considerable density of air molecules.

Which brings us to space and the near-vacuum at the edge of space.

We can say that, at 21 miles up, the atmosphere has ceased to exist although a few molecules move around without the density of the lower atmosphere.

Fewer molecules collide with each other way up there, so there's less heat. As a result, there's temperature without heat. And up where the space station orbits the Earth, one air molecule travels 43 miles before colliding with another molecule-compare that to a traveling distance of only four-millionths of an inch at sea level.

At 43 miles altitude, Kittinger wrote in a 1961 account about his stratospheric balloon adventure, "in the interval between collisions, each molecule gains tremendous velocity. Expressed as an air temperature reading this is 4,118 degrees Fahrenheit. It is a meaningless temperature, for there is no heat. Temperature definitions in space break down. They mean something entirely different. The temperature-with-heat condition depends upon exposure to solar radiation.

"If I had been completely in space," Kittinger added, "where there can be no temperature, and exposed directly to the Sun, I could have baked to a fine crisp on one side, and simultaneously frozen on the other."

Lou Varricchio, M.Sc., is a former NASA senior science writer and an active member of the NASA/JPL Solar System Ambassador program in Vermont. He is a recipient of the U.S. Civil Air Patrol's Gen. Charles "Chuck" Yeager Aerospace Education Achievement Award.

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