QUESTION:
How did you test the Pathfinder's airbags to make sure they held up during
impact?

ANSWER from Rob Manning:
We knew that we could only build Pathfinder and Sojourner to withstand about
50 times Earth's acceleration without breaking. Airbags we knew would be a
potentially cheap way to land on Mars (of course I am ignoring the
heatshield that slows the vehicle in the upper Martian atmosphere; the
parachute that does the same, the three solid rockets that brings everything
to a stop some 50 feet in the air before releasing the lander and rover to
bounce on the surface inside the bags.)

Well things are always easier said than done.

At first we tried to test the lander-airbag combination by dropping the
lander (actually a full scale mockup that weighs the same) from a tower at
Sandia National Lab in New Mexico. We soon realized that the unique
environmental conditions on Mars made those tests irrelevant. The most
important characteristic is the Martian "ambient" air pressure (the low
temperatures and the lower Martian gravity are of lesser importance).

On Mars the pressure is about 1% the pressure on Earth (Earth's is about
14.7 pounds per square inch or psi). The ability of the airbags to cushion
an impact depends on how much gas you put in it. On Earth (outdoors) you
need to put a LOT of air in the bags just so that they stay inflated (in
essence you need to "fight" the outdoor air pressure). If you want to have
them inflated to a nice stiff 1 psi pressure difference between the inside
and outside of the bags, you need to put 15.7 psi TOTAL inside the bags on
Earth. With so little air on the surface of Mars we didn't need to put
nearly as much gas in the bags to inflate them to 1 psi difference between
the inside and outside (nor did we want to, gas is heavy and we could only
lift so much weight to Mars from Florida). With so much less gas inside the
bags, the bags behave quite differently when dropped on Mars. Most
importantly, the bags stay nice and soft even if you squeeze them a lot
during an impact. Those same bags on Earth, on the other hand, become real
stiff and hard once you reduce the volume of the bags as a result of an
impact. The net result, is that the bags on Mars work a lot better and
provide a nicer and softer cushion than they do outdoors on Earth.

Well that's the good news, but how do you test it? The only way is to drop
the bags onto a rock surface at a place where the air pressure is the same
as on Mars. Where is that? Well inside a vacuum chamber where a pump could
suck all but 1% of the air out of. A BIG vacuum chamber! The biggest in the
world is in Sandusky Ohio and is run by NASA Lewis Research Center. It is
120 feet high and wide. All we needed to do was to build a "Martian" landing
site (including big nasty looking rocks) inside the chamber and drop our
airbag-enshrouded vehicle from the ceiling of the chamber. That's what we
did in 1995 and 1996. In fact to simulate landing with a large horizontal
(as well as vertical) velocity, we built a HUGE 60 by 20 foot platform with
sharp rocks BOLTED on and tilted UP 60 degrees inside the chamber! Once we
did that we realized that the lander wouldn't hit that platform with enough
speed so we also built a huge cable and bungie cord system that actually
PULLED the lander straight down onto this rock platform with an acceleration
of about twice that of gravity! This allowed us to simulate impacts up to 28
meters per second (about 60 MPH) which is faster that we expected the real
lander to actually hit the ground on Mars. (Pathfinder's first impact was at
about 14 meters per second and resulted in "only" 18 Earth gees of
acceleration).

After many failures of our early airbag designs, we (JPL and our friends at
ILC Dover Co in Delaware who built the bags) FINALLY in November of '95, we
got to the point where the bags would not break at all and the lander (and
of course Sojourner) would always be protected. And were we relieved!! Those
early tests where the bags nearly exploded really made us afraid that
getting to Mars with airbags might not work! We learned from our mistakes.
By the time Pathfinder launched in Dec of 1996 (about a year ago), those of
us involved in the testing felt quite confident that the airbags would
protect the lander.

Rob Manning