QUESTION:
When the Pathfinder lands, what if the door opens next to a crater or a
canyon? Will the Rover be able to stay right side up and either get out of
the crater or find some way of getting out? What if the petal on which the
rover sits happens to end up perched atop a rock/boulder, up above the
ground? If the rover cannot leave the lander, how seriously does this reduce
the amount of science that can be done?

ANSWER from Robert Manning on May 21, 1997:
Good question! There are two unfurlable ramps that the rover may use to
get off of the lander petal that it rides on on the way to Mars. If the lander
opens its petals next to a big rock, depending on which side of the rover petal
the rock is on, we may choose to tell the lander to only release the rover's
exit ramp on the side that is NOT obstructed by the rock. This means that the
rover may have to "back up" down the "back" ramp in order to get off of the
lander. There have been cases in our tests where the ramp can get pretty steep.
If the ramp is no steeper than around 40 degrees, the rover will be able to
drive down it without fear of tumbling off.

Once the rover is off of the ramp, it may be that the surrounding area is too
steep for the rover to climb up and out of. If that happens then we will tell
the rover to stay put and just try to analyze the soil that it is standing on.
If there is a rock nearby, we will tell the rover to analyze that rock too.
Although it would be very interesting to be able to send the rover out and away
from the lander, most scientists agree that just analyzing nearby rocks and soil
would be great. This is what we will do on the first weeks after Mars Pathfinder
lands. After that we hope to send it out away from the lander to explore the
surrounding area. It will probably never go further than 100 meters from the
lander; there is just too much to do!

ANSWER from Rob Manning:
Like you, we have envisioned situations where it is all but impossible for
the rover to get safely off of its petal. Although we tried, we could not
design it to work in all possible cases. The good news is that we believe it
to be very unlikely to get into the kind of situation you describe. We
believe that for the following reasons:

1. The geometry of the inflated airbags tends to push the lander away from
large objects - as the bags roll, they tend to "find" low flat areas
somewhat free of the largest boulders. In our tests in the Mars Yard at JPL
we found it quite difficult to get the lander (inside the partially inflated
airbags) to sit right next to a large boulder. We had to "cheat" by forcing
that situation manually.

2. Even when we did manage to get the lander to tilt way up so that the
rover petal was in the air (held by its petal actuator in a sort of "iron
cross"), we found that we could lower the petal below the "flat" position to
reduce the distance between the petal and the surrounding ground. Of course,
this tends to lift up the base "petal", but once the rover has made it down
the ramp we could raise that petal back up.

3. The rover ramps are a meter long and are flexible enough at the point
where they "hinge" so that they will go as far down (steep) as it can be
reached. The rover designers would like to avoid ramp inclines greater than
35 degrees which puts the top edge of the petal no more than about 0.57 m
(or a little below 2 feet) above the ground. Considering the density of 0.5
m rocks is very low, this is quite respectable. (They tell me that they
might consider letting the rover drive down a 45 deg ramp, but they would
have to hold their breath!)

If for some reason the rover fails to get off of the ramp, the consequences
for the mission are somewhat damped. The image data obtained by the IMP
camera and the weather data obtained by the ASI/MET would still provide a
lot of science data for future researchers to mull over for many years to
come. But it would be a sad loss to both the rover designers (who would very
much like to prove the concept of robotic roving vehicles on Mars) and to
the geologists who would like to get a better understanding of the elemental
composition of Martian rocks.

--Rob Manning, Mars Pathfinder Chief Flight Engineer