Re: Why can't a returning spaceship use a parachute?

Hi Kelly!  But returning spaceships DO use parachutes.  (Imagine the 
returning Apollo craft, suspended under its red striped triple chutes.)  You 
must mean, "Why can't spaceships use parachutes during the reentry phase?"  The 
problem is speed.  If I recall correctly, the speed for low earth orbit is 
around 10km per second, or 18,000 miles per hour.  At this speed, the air is 
compressed ahead of the spaceship and becomes as hot as a flame.  It also rubs 
against any extended parts of the ship and heats them red hot through friction.  
A block of composite ceramic material can survive this, but textiles cannot.

Why don't skydivers burn up?  Because they never move faster than a couple of 
hundred miles per hour.  That's several hundred times slower than a reentering 
space ship.

If things worked out proportionally, then a skydiver would suffer several 
hundred times less heating than a spaceship heat shield.  (I think things are 
actually less than proportional, so the skydiver is heated even less.)

One instance I have heard about where things do burn up even at ground level: 
ultra-high velocity cannons.  If a cannon fires a shell at many times faster 
than the speed of sound, the cannonshell will "burn down" and there will be 
nothing left of it by the time it reaches its target.