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 destructio