Space equavalency is said to begin at the Armstrong Line of the upper atmosphere located at 62,000 feet above sea level (11.8 miles). This imaginary line was first discovered by U.S. Air Force surgeon Harry George Armstrong. Pressure at the Armstrong Line is a mere 0.0618 atmospheres.
Today, some military and civilian jets travel to the edge of space-at or just above 70,000 feet in altitude. A few Russian MIGs, the Air Force's U-2, and NASA's ER-2 aircraft make occasional jaunts to this rarified place where the atmosphere isn't much to speak of. Here, the pilots are required to don fully pressurized spacesuits-just in case of cabin failure.
Exploration at the edge of space began in earnest during the 1950s and 1960s; research revealed some unusual aspects about temperature and heat in the near vacuum of the space-equivalent environment.
During his dangerous open-gondola balloon trip to 102,800 feet in Excelsior III in 1960, USAFCapt. Joe Kittinger uncovered an unusual phenomena. Outside his pressure suit, Kittinger noticed that the "air" temperature 21 miles up was minus 38 degrees Fahrenheit. Yet he observed that it was meaningless compared to how the rest of us define temperature back on Earth. At the edge of space, Kittinger experienced, there is a strong difference between temperature and heat-they are, in fact, very distinct from each other.
Temperature is a measure of motion-the motion of air molecules. When we stand at the beach, at sea level, air molecules are at their densest-the entire column of Earth's atmosphere pushes down on them (and us) from above. Without getting into the details of how, a researcher measured just how many molecules exist at sea level in a cubic inch of air. The answer is an incredible 400 quintillion molecules!
At sea level, air molecules are like ballroom dancers on a very crowded floor. They are elbow-to-elbow and don't have much room to demonstrate their Tango or Fox Trot moves-in fact, air molecules constantly collide at sea level being a mere four-millionths of an inch apart. The velocity of these air molecules bouncing off each other is slowed down thus lowering their temperature.
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.