QUESTION: 
I want to know about the heat shields on the spacecraft.  
1. What would happen if the spacecraft didn't have heat shields? 
2. Would the spacecraft totally evaporate if the heat shields fell off 
   (or weren't there in the first place), or would pieces of the spacecraft 
   fall to Mars?  

ANSWER from Pieter Kallemeyn on January 30, 1997:
If Mars Pathfinder did not have a heat shield, the spacecraft would 
not completely evaporate during atmosphere entry, but instead breakup 
into smaller pieces and hit the surface as charred and melted chunks 
of metal.  In fact, this is exactly what happens with  Mars 
Pathfinder's cruise stage, which is a part of the spacecraft that 
contains the solar array, thrusters and fuel tanks, (but does not have 
its own heat shield).  The cruise stage is used during the 7 month
flight from Earth to Mars, and then separates from the lander 35 
minutes before landing.  Our studies have shown that most of it will 
land on Mars in charred pieces.

Why doesn't the cruise stage have it's own heat shield?  Because once 
we finish the journey from Earth to Mars, the equipment on the cruise 
stage is not needed once we get to the surface, since the lander has 
it's own power system.  So it's okay to throw it away, knowing that 
the heat of atmosphere entry sterilizes the pieces so no biological 
contamination of Mars can happen.

To see a picture of the cruise stage, check out:

            http://mpfwww.arc.nasa.gov/mpf/pictures.html