First wheels on Martian soil

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On sol 2, the command was sent to extend Sojourner to its full height and release its wheels from the lander. Drawing power from its 0.25 square meter solar panel and responding to commands from JPL, the rover drove down the ramp onto the

Martian surface at 05:40 Universal Time (UT). The first thing the rover did was to lower its APXS to the red soil for its first analysis. Sojourner's deployment onto the Martian surface, put together from a series of still images, was already being patched to networks and would become the top story for newscasts around the world. Images from the area around the lander were used to select the activities for the rover on sol three. At the Experiment Operations Working Group planning meeting for the next sol's events, the images displayed a nearby rock that would be the target of Sojourner's efforts.

Discussions had gone on at JPL regarding the identification of surface features during the mission. The only restriction issued by Dr. Golombek was that no names of actual individuals, either living or dead, could be used, so the target rock for sol 3 was thus identified as Barnacle Bill. The rover team was by now also gaining experience in "driving" the rover, traveling at one centimeter per second, giving them plenty of opportunities to study other potential targets. By the end of sol five, Sojourner had achieved its primary mission goals, two days ahead of schedule. It also returned a superb image of the Pathfinder lander. The health of the rover and the lander were excellent, and the mission now went into "overtime," officially known as the extended mission. The public fascination with the Mars Pathfinder mission was overwhelming, as the JPL website that gave mission status reports and images from the lander and rover experienced nearly fifty million hits in its first week.

Sojourner, in its stowed position, was secured to one of Pathfinder's solar panels. White room conditions were essential during all aspects of the spacecraft's preparation and closeout prior to launch. (NASA/JPL-Caltech)

The rover visited other rocks or areas of intriguing soil over the following two weeks, among them Yogi, Scooby Doo and Cabbage Patch. As whimsical as the names were, the rocks and soil held a treasure trove of much desired knowledge of Mars and its history. Images from the lander's IMP and the rover's APXS were confirming the predicted findings for this region of Mars. On sol thirty-five, Sojourner headed for a dense rock field dubbed The Rock Garden. The APXS was brought to bear on rocks named Moe, Stimpy, Half Dome and Shark, among others. The rover also sent back close-up images of these rocks. It was clear by now how these rocks had been deposited on the Ares Vallis plain. On the Mars Pathfinder website, the scientific team stated:

"The mosaic of the landscape constructed from the first images revealed a rocky plain (about twenty per cent of which was covered by rocks) that appears to have been deposited and shaped by catastrophic floods. This was what we had predicted based on remote-sensing data and the location of the landing site (19.13 degrees north, 33.22 degrees west), which is downstream from the mouth of Ares Vallis in the low area known as Chryse Planitia. In Viking orbiter images, the area appears analogous to the Channeled Scabland in eastern and central Washington state. This analogy suggests that Ares Vallis formed when roughly the same volume of water as in the Great Lakes (hundreds of cubic kilometers) was catastrophically released, carving the observed channel in a few weeks. The density of impact craters in the region indicates that it formed at an intermediate time in Mars's history, somewhere between 1.8 and 3.5 billion years ago. The Pathfinder images support this interpretation. They show semi-rounded pebbles, cobbles and boulders similar to those deposited by terrestrial catastrophic floods. Rocks in what we dubbed the Rock Garden - a collection of rocks to the southwest of the lander - are inclined and stacked, as if deposited by rapidly flowing water. Large rocks in the images (0.5 meters or larger) are flat-topped and often perched, also consistent with deposition by a flood.''

The Pathfinder lander had a combined solar array from its three petals of 2.8 square meters, with rechargeable silver-zinc batteries. The rover ran essentially on its solar panel but had lithium thionol-chloride D-cell batteries as backup and auxiliary power. On sol fifty-six, its batteries ceased giving power, so the little rover could now only function during the Martian "day." Data from the lander's batteries showed their capacity was also slowly deteriorating. "Martian Time'' as it was referred to, was also having a debilitating effect on the small team of rover "drivers", the engineers who commanded Sojourner. Over a period of two months, the engineers had to report to JPL at always shifting times, to the point that their lives had no correlation to daytime and nighttime on Earth. It was fatiguing and tedious and some engineers simply forgot the scientific and space exploration history they were making. Andrew Mishkin had to remind himself of the significance of what the rover team had accomplished, and what the mission meant to him.

"I think the most gratifying results were in getting to see things that could not be seen from the lander,'' Mishkin stated. "For example, sand ripples behind rocks the lander could not see, and getting close-up views of cracked or shattered rock. My personal interest was mostly in the ability to vicariously explore.''

As the mission passed its eightieth sol, it had become abundantly clear that the JPL team had engineered a very rugged lander and rover. Some, however, speculated just how long it could go on. Finally, on the eighty-fourth sol, 27 September 1997, the Mission Support Area lost contact with the lander. Repeated efforts to reestablish contact failed and JPL mission management declared the mission completed. Mars Pathfinder had returned 2.3 billion bits of information, which included over 16,500 images from the lander and 550 images from the rover. Sojourner had provided fifteen chemical analyses of rocks and soil. Perhaps just as impressive were the more than 750 million "hits" the JPL Pathfinder website had recorded during the previous three months. The mission had been a spectacular success and JPL commenced planning for the next generation of Mars rovers for even more ambitious missions.

The loss of Pathfinder and Sojourner was overshadowed by the arrival two weeks previously of the Mars Global Surveyor probe that was now circling the planet. This was a sophisticated orbiter that would take unprecedented images and perform remote sensing and mapping. The success of these two missions gave added momentum to those planned and already underway, but as history would prove, this was no guarantee that the next missions would succeed.

Pathfinder landed in the Ares Vallis region of Mars on 4 July 1997. Two days later, Sojourner drove down the ramp onto the Martian surface. Nominal mission length was seven Martian days, or sols, but Sojourner continued to operate successfully for more than 80 sols. (NASA/JPL-Caltech)

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