Just one of the many problems in landing on another planet, after it’s been
determined where to land and the method to get there, is landing safely. For
JPL, a safe landing is "the name of the game," as engineers work to prepare
two rovers for the journey to Mars.

The Mars Exploration Rovers scheduled for launch in 2003 are using the same
type airbag landing system that Mars Pathfinder used in 1997. The airbags
must be strong enough to cushion the spacecraft if it lands on rocks or
rough terrain and allow it to bounce across Mars’ surface at freeway speeds
after landing. To add to the complexity, the airbags must be inflated
seconds before touchdown and deflated once safely on the ground.

"The 2003 rovers have a different mass [than Sojourner, the Pathfinder
rover], so we’ve made changes in the airbag design," said John Carson,
cognizant engineer. "Our requirement is to be able to land safely on a rock
extending about a half-meter (about 18 inches) above the surface. Extensive
testing gives us a process for trial and error before the final design."

How to Build a Better Airbag

While most new automobiles now come with airbags, spacecraft don’t. The
fabric being used for the new Mars airbags is a synthetic material called
Vectran that was also used on Mars Pathfinder. Vectran has almost twice the
strength of other synthetic materials, such as Kevlar, and performs better
at cold temperatures.

Denier is a term that measures the diameter of the thread used in the
product. There will be six 100-denier layers of the light but tough Vectran
protecting one or two inner bladders of the same material in 200-denier,
according to Dara Sabahi, mechanical systems architect. Using the 100-denier
means there is more actual fabric in the outer layers where it is needed,
because there are more threads in the weave.

Each rover uses four airbags with six lobes each, which are all connected.
Connection is important, since it helps abate some of the landing forces by
keeping the bag system flexible and responsive to ground pressure. The
fabric of the airbags is not attached directly to the rover; ropes that
crisscross the bags hold the fabric to the rover. The ropes give the bags
shape, which makes inflation easier. While in flight, the bags are stowed
along with three gas generators that are used for inflation.

Testing, Testing, Testing

Since the airbags are composed of many layers, some tearing in the outer
layers is acceptable and even expected. Engineers test the bags to make sure
there will be no catastrophic problems that would prevent a safe landing.

Mars airbag testing is done in world’s largest vacuum chamber at the Plum
Brook Station of NASA’s Glenn Research Center in Ohio. "The Plum Brook
facility is pretty impressive, along with all the people who operate it,"
said Carson.

The test chamber used for the tests is a little over 30 meters (100 feet)
across and about 37 meters (120 feet) high — big enough that three railroad
tracks go through it. A test spacecraft and airbag system weighing about 535
kilograms (about 1,180 pounds) are accelerated with a bungee cord system
onto a platform with rocks that approximate the Mars surface. The drop is at
landing speed, about 20 to 24 meters (yards) per second.

Tests are documented thoroughly with high-speed and video cameras, in
addition to visual inspections. Engineers even built a clear dome, studded
with rocks, that has a camera that documents tests from a rock’s-eye view.
During testing, a crew from ILC Dover, the airbag’s manufacturer, stands by
to make quick repairs and to note any changes required.

"We do extensive testing," said Tom Rivellini, deputy mechanical systems
architect. "We want to break the bag on Earth, not on Mars. If we see a tear
that is unexpected or goes too deep, we can make changes now [before the
final design]."

Carson added, "We’ll go over all the data we’ve accumulated so far, do some
more testing, and decide on a design configuration."

And then on to Mars in 2003!