BOX 3.2 Sojourner
Sojourner, the rover on the Mars Pathfinder mission (Figure 3.2.1), was basically a rectangular table on wheels (Table 3.2.1). The "tabletop" was a 0.25 m2 solar cell array. Navigation was controlled from Earth, but the rover was capable of avoiding obstacles by using laser sensors and a simple avoidance protocol included in its on-board processor (Intel 80C85). Navigation was accomplished with two forward-looking monochromatic cameras that provided a stereoscopic view of the terrain. Sojourner's maximum speed was 1 cm/s for a total possible range of about 60 m/day, assuming continuous driving and no obstacles; actual speeds and distance traveled were much less. The maximum data relay rate was - 30 Mbits/day, via the lander.
The suspension system allowed individual wheels to climb over obstacles as high as the wheel diameter (13 cm). In addition, because each wheel was independently driven, Sojourner could use them to dig shallow trenches in the martian soil. A color monoscopic camera was mounted at the back of Sojourner, along with an alpha-proton x-ray spectrometer (APXS). The latter was mounted on an arm that permitted placing its sensor against either rocks or soil. A full elemental analysis of a particular sample required about 10 hours. The color camera imaged the spot where the analysis was carried out. The chemical data returned included some surprises, such as rocks with sufficient SiO2 that quartz appears in their calculated "normative" mineral compositions, and with higher SiO2 than any of the known martian meteorites or any martian soils.
Sojourner was originally designed to conduct a 7-day mission on the martian surface. After about 60 days, the batteries were depleted but the rover continued to operate during daylight hours by using solar power. The last contact with Pathfinder was on October 7, 1997. Loss of contact is believed to have been due to extremely low temperatures in the lander. Despite a few communication problems and minor glitches with the positioning of the APXS, Sojourner performed exceptionally well, far exceeding design expectations.
TABLE 3.2.1 Characteristics of Sojourner