A spacecraft returning from the Moon and reentering the Earth's atmosphere far exceeds the speed of the ballistic missiles that "leisurely" fall planetward from the fringes of the atmosphere. When spacecraft heading home from the Moon impact the atmosphere,  it is like hitting a fiery wall. Temperatures above that of the surface of the Sun prevail around the exposed forward surfaces. These spacecraft are, in essence, artificial meteors; and it is common knowledge that natural meteors are mostly consumed in their whitehot descent through the atmosphere. To design the spacecraft heat shield, terrestrial wind tunnels are used to simulate flow conditions characteristic of reentry speeds in the neighborhood of 37 000 feet per second for lunar reentry and 50 000 feet per second and above for planetary reentry. If wind tunnel simulation were to prove impossible, the spacecraft designer could never be certain that a fatal error in design concept or some undiscerned flaw in the heat shield might lead to the destruction of the vehicle. To preclude such a grave consequence, rocket-launched unmanned flight vehicles are used, where practicable to validate the integrity of the vehicle design.
The term hypersonic has been used to define the speed regime above about Mach 5, at which heating of the air becomes an overriding factor in vehicle design. In hypersonic wind tunnel operation (below Mach 10 with gas heated to prevent liquefaction), it is assumed that the air streaming by the body behaves as a perfect gas, as defined by the laws of thermodynamics. However, as space vehicles progress into the regime of orbital entry speeds, the strong shock wave generated near the nose of the body produces a very large temperature increase (and a pressure increase as well) that will change the chemical composition of the streaming air. These are often referred to as "real gas" effects. The oxygen and nitrogen molecules in the air tend to dissociate and may become electrically charged and form an ionized sheath around the entry vehicle. This sheath can block the transmission of electromagnetic radiation. The dramatic communications blackout experienced by the first Mercury capsule during reentry illustrates the phenomena. This is called the regime of hypervelocity flight. To reproduce this group of extreme conditions in terrestrial laboratories, aerodynamicists have designed exotic facilities that are usually called wind tunnels, but that stretch the definition considerably.
A fact of life faced by the designer of a very high speed wind tunnel is the extreme temperature of the air entering the nozzle that accelerates the air to the desired speeds. Just before the nozzle, in the stilling chamber, the wind tunnel air is essentially at rest. After accelerating through the nozzle and impacting the nose of the spacecraft model in the test section, the air is once again at rest. Since no energy has been added between these two stations, the temperatures of the air at both stations will be the same. Now a spacecraft entering the Earth's atmosphere at, for example, Mach 10, will experience a stagnation air temperature at the nose of approximately 8000 ° F. The implication for wind tunnels is that somehow the air in the stilling chamber must be heated to 8000° F (or even higher for higher velocities) to reproduce stagnation reentry temperatures on the model. Such temperatures approach those of the Sun's surface and far exceed those normally available in industrial and scientific laboratories.