In Chapter 1, the four basic environmental interactions were introduced. To help in understanding the physics of these interactions with a spacecraft, the nondimensional physical parameters that determine the interactions are described in Chapter 2. An understanding of the magnitudes of these nondimensional parameters simplifies the complex physics describing the interactions. This is analogous to the idea that, for a fluid, the definition of a Reynolds number allows the physics of the fluid behavior to be divided into two regimes. When the Reynolds number is small compared to unity, the physics of the fluid is dominated by viscous effects. When the Reynolds number is large compared to unity, the flow is inviscid. Likewise, for a compressible gas, the Mach number allows the physics of the flow to be divided into subsonic physics and supersonic physics. This idea of fundamental scales, like the Reynolds and Mach numbers, is exploited in this and subsequent chapters as a means of characterizing the effects and importance of the fundamental interactions under varying environmental constraints.