When Mars Pathfinder landed on Mars, it relied on air bags much like those in automobiles to cushion its landing. NASA Glenns' Plum Brook Station in Sandusky, OH, hosted tests of the innovative air bag landing gear system that enabled Pathfinder to bounce off the rugged terrain of Mars.

Air bags underwent drop testing on a simulated Mars terrain in the Space Power Facility at Plum Brook Station in 1995. Credit: NASA
The Surveyor spacecraft were the first U.S. vehicles to achieve soft landings, by using retrorockets firing intermittently toward a planet's surface. The Viking Martian landers likewise used retrorockets. While retrorockets are effective, they are known to leave trace amounts of foreign chemicals on the surface. Since one objective of this mission was to analyze the elemental composition of Martian rocks and soil, another method of landing had to be developed to avoid contaminating the samples.

The $4 million NASA-designed air bag landing gear system, manufactured by ILC Dover, Frederica, DE, was composed of 24 interconnected spheres that protected the Mars Pathfinder spacecraft and its delicate payload composed of a lander, camera, and instrument-laden rover upon impact with the Martian surface. The pyramid-shaped air bag system, which measured 17 feet tall and 17 feet in diameter, was fabricated of materials similar to those used in space suits.

The air bag system underwent a series of rigorous tests in Plum Brook's Space Power Facility, the world's largest vacuum chamber pressurized with a simulated Martian atmosphere. When the Mars Pathfinder spacecraft entered the Martian atmosphere, it was traveling about 17,000 miles per hour. A parachute and rocket braking system slowed the spacecraft to about 50-60 miles per hour. Then the air bags deployed to protect the spacecraft when it impacted the surface. The Space Power Facility enabled the project team to test the air bag system in the extreme atmospheric conditions it was subjected to when it reached Mars.

Air bags were inflated during testing in the B-2 facility at Plum Brook. Credit: NASA
The air bag system was tested on numerous terrains ranging from flat surfaces to steep, rocky inclines. While initial single-layered designs could not survive the jagged rocks, testing enabled the JPL team to concoct the perfect recipe for unbreakable, lightweight bags: multiple layers of lighter fabric instead of a single layer of heavier material. The outer layer was tearing, absorbing energy, but more importantly creating a buffer, like a soft blanket over the rocks. The second layer didn't get all the sharp little edges and corners digging into it.

The final air bag design was then tested in Plum Brook's Spacecraft Propulsion Research Facility (B-2) to ensure proper inflation. Packed tightly together into a pillow sized compartment, the air bags were inflated in the -80 degree Celsius vacuum facility. The bags fully inflated in half a second, while gas generators maintained a constant pressure inside the bags. Similar tests conducted on the retraction of the bags also proved successful.

With the overall mission dependent on the proper operation of the air bags, the Plum Brook tests played an important role in the risk reduction process. It is likely that air bag landing systems will become a viable alternative for all future small, unmanned planetary missions.