Soft Landing Achieved by Pathfinder
Ares Vallis, Mars — On July 4,l997, all went well, and Pathfinder and its precious cargo of high-tech instruments landed here with a bounce, not a crash. To accomplish this, mission engineers had preprogrammed a sequence of automated steps and equipped the spacecraft with some ingenious devices to take full advantage of the thin atmosphere of Mars. This landing sequence was risky and had never been tried before.
Unlike the Viking landers, Pathfinder did not orbit the planet before landing. Instead, it entered the atmosphere at a low angle, traveling about 8 kilometers (5 miles) per second. Enclosed in a capsule in which it traveled from Earth, Pathfinder plunged through the thin air of Mars as a dual-purpose heat shield protected it from blistering temperatures produced by the friction and slowed it down to about 1/20th of its entry speed—to about the speed of a supersonic jet.
When the capsule was several kilometers above the surface, a giant parachute burst open to slow the fall even more—to about the speed of a race car. Shortly thereafter, the heat shield, having done its job, fell away, as the capsule descended toward the Martian surface.
The capsule, shaped like a tetrahedon, was equipped with an airbag system, which inflated like a cluster of balloons about 300 meters (1000 feet) above touchdown. Like the airbags in an automobile, these were intended to protect the lander from damage when it landed by reducing the abrupt change of speed on impact. They were made of a super-strength fiber (the same as used in bullet-proof vests) to resist puncture if they hit any sharp Martian rocks.
Approaching the surface, three small retrorockets fired, bringing the lander to a dead stop in midair within two seconds. The rockets then carried the parachute and back end of the capsule out of range. That kept the parachute from falling on the lander and the landing site was not contaminated by rocket exhaust.
Finally, the lander, surrounded by the airbags, dropped the last few meters to the ground. Everything worked as planned, Pathfinder bounced and rolled like a child's rubber ball, and eventually came to a stop.
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