QUESTION:
When the airbags land on Mars, won't the bags pop because of the 
rocks that are scattered around on Mars' surface?

ANSWER from Cathy Davis on January 14, 1997:
If the rip is small enough, smaller than about a foot, the airbag will
function normally.  If the rip is larger than about a foot the airbag
could lose enough air from the inside that the lander could get
damaged.  If there was a terrible problem and the airbags received a
very big hole, the lander could possibly get destroyed, but even with
a huge hole in the airbags, they will still protect the lander a
little bit.

ANSWER from Guy Beutelschies on February 19, 1997:
We built the bags using vectran, which is very tough.  It is a lot like the
material they use to make bullet proof vests.  When you touch the bags,
they feel like very thick canvas.

In spite of this, we were still worried about them ripping on rocks so we
tested them by dropping them inside a very tall vacuum chamber onto a
platform where we had mounted very sharp rocks.  The first tests showed
that they did rip!  We went back to the drawing board and added another
loose layer on the outside of the bag that would help protect the inner
bag.  We tested this and it worked much better.

The tests also showed that even with a small tear, the lander would still
be safe because it would stop rolling and bouncing before all the gas could
escape.

Guy Beutelschies:	Mars Pathfinder Flight Engineer

ANSWER from Rob Manning:
There is a possibility that the airbags will hit a sharp rock. Although our
Mars geologist claimed that that is a low probability, we nevertheless want
to make sure that our bags could handle a large sharp nasty rock even under
the worst (fastest/shallowest) impact condition. We knew that very large
rocks (even sharp ones) were not a problem because they tended to act like
small hills. There is sort of a "magic" size of sharp rock which could do
the maximum damage. We also knew that "needle" rocks (like the tufa towers
near Mono lake in California) are highly unlikely even on foreign planets
like Mars - there are only just so many ways that rocks can be formed. From
experiments and geometry (the lobes of the bags are 1 meter in diameter) we
decided that the worst sharp rock was a 0.5 to 0.7 meter sharp rock point
that is firmly attached to the Mars ground. So how do you make sure it can
handle such a rock? Test it! We built a large sloped platform inside the
Plum Brook 120 ft vaccum chamber in Sandusky Ohio and we studded it with
sharp 0.6 m rocks (the nasty kind that can rip skin). These rocks were
bolted down to the ramp and a full scale heavy lander inside the bags was
pulled down with bungy cords so that it impacted these rocks at better than
25 m/s (60 mph).

After some trial and error over several months in late 1995 and early 1996
(redesigning bags as we went) we found that the bags would take this abuse
with minimal damage and only minor puncture. It turns out that vectran bags
do not "pop" when they are punctured. The incredible tensile strength of
this fabric (and the special "rip-stop" weave) prevents propagation of
tears.

--Rob Manning