Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter the earth's
atmosphere at high speed. Isn't there some way to come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled the idea of
unfolding some large wings to add a lot of surface area, or using
propellers to resist falling, or parachutes? Thank you.

A spacecraft enters the atmosphere at such high speed because its
orbital speed is so high to begin with. On orbit a satellite has a
certain (large) orbital speed, and if some of that speed is lost, for
instance by firing its rocket motor forward, then the shape of the
orbit changes so that it dips downward, closer to Earth. If the
satellite loses enough speed, then the path dips right down deep into
the atmosphere. This is, in fact, how a returning spacecraft is made to
lose enough of its speed to allow it to land safely, by plowing through
the atmosphere.

One may ask, "So why not just lose more speed before even entering
the atmosphere, eliminating the need for heat shielding?" First, think
of how much energy the spacecraft loses by its passage through the air.
In the case of the space shuttle it goes from the neighborhood of
17,000 mph to only a few hundred mph by aerodynamic drag alone. That's
nearly as much change in orbital speed as it aquired at launch. But
launch required an enormous quantity of propellant. In order to lose
all that speed by rocket would require the shuttle to have onboard
nearly as much propellant as it took to get it aloft to begin with.

But that original investment of propellant was just enough to get
the shuttle to orbit *without* all that extra fuel. To place it on
orbit with enough fuel to lose all or most of its orbital speed would
then require VASTLY more propellant at launch. It's quite possible it
couldn't even be gotten off the ground. The most practical & economical
method is for the shuttle to have just enough propellant available to
change its orbital path to dip down into the atmosphere, whereat it
loses most of its speed by drag.

Keeping this in mind, it's possible to see that folding wings won't
do much in the spacecraft's favor. The shuttle would still have to plow
through the air at the same speed, and in fact extra large wings might
even be a liability, since they'd increase the lift, keeping the ship
from dropping downward. All the extra wing area would do is increase
the orbiter's weight at liftoff, reducing the payload it can carry.

-Mark Martin