Why do spacecraft have to enter the earths atmosphere at such high speeds?
The problem is fuel. In space, there are no brakes, nothing to
slow you down - except the thrust of your own rockets. Fuel is
scarce and precious (like everything else in space) and at the end
of the trip, there isn't any left. So the only way to slow down is
to skim lightly through the upper layers of the atmosphere, and
allow friction between the spacecraft and the upper atmosphere to
slow you down.