QUESTION:
What would happen if the Pathfinder landed in the wrong spot?  What would 
happen if the Rover got stuck on Mars? How would you get it unstuck? How do 
you know that the rover won't fall off a cliff? Could the rover ever run out 
of energy and break down? What would happen if the solar panels cracked or 
were chipped?  
 
ANSWER from David Mittman on May 9, 1997:
It is unlikely that we'll land outside of our 100 x 200 kilometer target
area. As of May 9, 1997 the spacecraft is on a trajectory which would
cause it to land just short of our target (we'll be correcting our
trajectory two more times before landing).

If we were to landing very far away from our designated target, then we would 
need to re-plan some of our activities to account for less solar power and a 
different terrain. If this new landing area proves rockier, then Sojourner 
may move more slowly.

That depends on how it is stuck. The rover has six wheels, each powered by its 
own motor. Four of the wheels can be steered. The rover is a very capable 
off-road traveler. Should one wheel get stuck in a dust pit, for instance, the 
rover would be able to use the remaining five wheels to back up.

The rover uses a hazard avoidance system to detect changes in the
terrain in front of it. A "cliff" would be detected by the hazard
avoidance software and the rover would stop its forward motion.

To detect hazards, the rover takes two sets of stereo pictures. The
first set includes light stripes "painted" on the surface by on-board
lasers. The lasers are turned off for the second set which has no
stripes. The two sets of pictures are then "differenced" to leave only
the stripes. If the stripes are straight and extend completely across
the image, then the terrain in front of the rover is flat. If the
stripes are crooked, then the terrain is rocky. If the stripes are
straight but only extend half-way up the image, then the ground has
"fallen away" at that point, and the rover won't travel over that cliff.

The rover gets most of its energy from the Sun drawing upon its
batteries only occasionally. We don't expect to use up the rover's
batteries before the end of the rover's mission. We do expect the rover
to eventually break down, however. The temperature changes between day
and night on Mars by over 200 degrees (Fahrenheit). These temperature
changes will eventually cause parts of the rover to fail.

Each time a solar cell on the solar panel gets cracked or chipped, the
power available to the lander (and rover, which has its own solar
panels) goes down. The degradation of the solar panels has been factored
in to how we operate the spacecraft. We don't expect solar panel
degradation to have much of an impact during the first 30 days of our
surface mission.