The Mars Pathfinder program was an example of concurrent engineering under a pressing schedule and immovable budget. The whole spacecraft was, in fact, a series of complex systems, including the rover, the lander, the landing system and the cruise stage that would take the spacecraft to Mars after being launched aboard a Delta II rocket. Most of the Mars Pathfinder mission was cutting edge - it had never been done before. The landing system was a prime example. This mission would employ a unique (up to that point) system of heat shield entry, parachute deployment to slow the stowed spacecraft, backshell separation, lander separation and bridle deployment to distance the lander from the backshell, airbag landing system deployment, backshell retro rocket ignition to slow decent even further, and cutting of the bridle to permit the inflated airbag-protected lander to bounce across the landing site until
coming to a stop. The airbags would be deflated, and the lander would right itself by deploying its petals, one of which held the rover. Practically every system had a series of nerve-wracking development problems that had to be resolved to get it to where it would perform reliably. It was no exaggeration to say that every engineer had to perform his specific tasks to near perfection for the entire mission to succeed.
The rover's electronics were perhaps the most sensitive to the temperature extremes that would be experienced during the Martian day (which is roughly 24 hours and 39 minutes long). Extra care had to be made to insulate the electronics without unduly adding weight to the vehicle and this proved particularly difficult for the engineers at JPL.
"One of the greatest challenges was designing the system to keep the electronics warm, and to do that with a small enough mass,'' Mishkin said. "The original design for what we called our Warm Electronics Box to contain the electronics, batteries and all, was designed to keep the electronics from cycling through such a high temperature range that it would eventually, through thermal expansion, start to cause components to fail. The design of that Warm Electronics Box was very challenging because the mass was just too great initially, and it took some very creative work by some people on the mechanical team to get to the notion of using solid silica aerogel to provide the insulation at a very low mass. That took over a kilogram out of the weight of the rover. For a vehicle that weighed only ten kilograms or so, that was a huge win.''
Rocky 4 served as the development vehicle for the rover's systems, but two more critical vehicles would follow. The first was the System Integration Model (SIM). The SIM was practically identical in every way to the Flight Unit Rover (FUR), but the SIM would be the qualification unit that underwent all the necessary tests to validate the vehicle for the mission, much as the LRV Qualification Unit did for the flight unit LRV for Apollo 15, 16 and 17. Many of the tests planned for the SIM were much more severe than the pre-flight tests for the FUR or the actual conditions the rover would experience during the mission. A series of centrifuge tests of the SIM culminated in one with the centrifuge operating at 130 revolutions per minute, pushing the SIM to sixty-six times normal gravity - far more than the tests for the FUR. With all the results from the tests of the SIM correlated, work began on assembling the FUR. When it was finished it went through its own series of tests, as well as integration and further testing with the Pathfinder lander. JPL had conducted a contest during 1995 to name the future Martian rover, and the top two names selected were Marie Curie and Sojourner.
In mid-August 1996, the Pathfinder lander was carefully closed up, secured inside a protective shipping container, and trucked from Pasadena, California to the Kennedy Space Center in Florida. Two weeks later, Sojourner followed. It was secured in its own JPL-designed shipping container, nicknamed "the sarcophagus", which was then secured inside a commercial aircraft cargo container. This was delivered to the airport, where JPL engineers watched it being loaded on the jet bound for Orlando, Florida, before getting on the plane themselves. Arriving safely in Orlando, the rover's special shipping container was removed from the cargo container, loaded on a truck and driven to the Kennedy Space Center. For the next several months, Pathfinder and Sojourner underwent testing and integration, before the lander containing the rover was finally closed up for the last time and the fully assembled spacecraft was mated to the Payload Assist Module (PAM-D) in the Spacecraft Assembly and Encapsulation Facility (SAFE-2) building at Kennedy Space Center. It was then placed inside the Delta II-7295 launch vehicle payload fairing on 21 November 1996.
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