Into the Wild Blue Yonder
The Earth's gaseous atmosphere is an essential part of the biosphere of the planet. It provides the O2 required by most forms of life, a stable thermal environment, and protection from the harmful effects of cosmic radiation. Only the very closest atmospheric region to the surface of the Earth, which extends from sea level to an altitude of about 16 kilometers (10 miles), can support aerobic life. This is a mere 3% of the complete atmosphere of the planet. The thinness of the air above this life-sustaining envelope, however, has not prevented humans from testing their limits against it.
By convention the atmosphere is divided into a series of layers, or concentric shells, characterized primarily by their thermal properties. This structural arrangement, known as the International Standard Atmosphere, divides the atmosphere into five regions: the troposphere, the stratosphere, the mesosphere, the ionosphere (thermosphere), and the exosphere (Fig. 16.1). These regions are separated by theoretical boundaries known as pauses.
The atmosphere near the surface, the troposphere, which extends from sea level to an altitude of 16 kilometers, can support life. The air in the troposphere be-