QUESTION: 
If a rover were to come into problems, how would the situation be
fixed?

ANSWER from Chris Salvo on February 18, 1997:
The possibility of things breaking is of great concern to us.  Through 
the whole design and construction of the spacecraft and into 
operations, we try to think about all the things that could go wrong 
and consider how we might react.  On many past spacecraft, the 
designers chose to put two of nearly everything so that if something 
broke (like the radio or the computer) there was another one on the 
spacecraft that could be used instead.  
    
On the Pathfinder spacecraft (and the rover Sojourner) the constraints 
on the project to be very low cost forced us to think differently.  We 
could not afford to put two of everything so most of the spacecraft 
and the rover is what we call "single-string."  This means that there 
is only one of each component.  So, if we only have one of each 
component, what do we do if something breaks?  The answer is we try to 
figure out how to "fix" the thing that broke, or we try to live 
without it.
     
How do we fix something that has broken?  It is possible that it is 
not completely broken, but that it just doesn't work quite right 
anymore.  I remember when I was about 10 years old we had a TV set 
that "broke" in a way.  When you turned it on, instead of showing you 
the whole picture, it would just show a bright dot in the middle of 
the screen.  Broken, right?  No, not really.  If you waited for about 
15 minutes, eventually the dot would grow into the whole picture just 
like there was never anything wrong.  As long as you turned it on 15 
minutes before you wanted to watch anything, it was ok.  Sometimes 
similar things happen to parts of spacecraft, and we can figure out 
how they work and use them anyway, even though they don't work how we 
designed them to work.
     
Sometimes we are not so lucky and things really break for good 
(eventually that's what happened to the TV I told you about.  It got 
to so bad that the dot never grew into the whole picture).  The only 
choice we have then is to try to live without the thing that is 
broken.  If it is a really critical part of the spacecraft like the 
central computer, and it really is broken then the mission is over.  
Sometimes, though, we can learn to do without the thing that broke.  
For example, let's say that you're in your car and you go to open the 
door and it doesn't open.  In fact, none of the doors will open.  All 
the doors are broken and cannot be opened.  Is this the end?  How else 
can we get in and out of the car?  What about the windows?  We could 
open the windows and climb in and out of those to get in and out of 
the car.  That would not be very convenient (and when it rained the 
car would get wet inside), but if it is the only car we have and we 
can't fix the doors then climbing in and out of the windows would have 
to do.
   
The same thinking applies to the spacecraft and the rover.  Let's say 
one of the motors on one of Sojourner's six wheels broke.  The other 
five wheels still work, so the rover could "limp" around Mars by 
dragging the wheel that is broken.  It would not be able to climb over 
things as well, but it could still move around.  
  
     Chris Salvo