Sojourner Rover
Sojourner, the small rover onboard Mars Pathfinder, was named after an African American reformist, Sojourner Truth, who lived during the tumultuous era of the American Civil War and made it her mission to “travel up and down the land” advocating the rights of all people to be free. The name was chosen by a panel of judges from the Jet Propulsion Laboratory and the Planetary Society following a year-long, worldwide competition in which students up to 18 years old were invited to select heroines and submit essays about their historical accomplishments. Sojourner Truth was shortened to Sojourner because it means “traveler.”
Sojourner with its mounting and deployment equipment weighs about 17.5 kilograms (38.5 pounds) at launch. Once it is mobile and operating on the Martian surface, it will weigh a mere 10 kilograms (22 pounds). The vehicle travels 1 centimeter (0.4 inch) per second and is about 65 centimeters (2 feet) long by 48 centimeters (1.5 feet) wide by 30 centimeters (1 foot) tall. During the cruise to Mars, it will be folded in its stowage space and measure only 18 centimeters (7 inches) tall.
Equipped with three cameras — a forward stereo system and rear color imaging system — the Sojourner rover will take several images of the lander to assess the lander’s health.
Sojourner is powered by a 0.2-square-meter (1.9-square-foot) solar array, sufficient to power the rover for several hours per day, even in the worst dust storms. As a backup and augmentation, lithium sodium dioxide D-cell batteries are enclosed in the rover’s thermally protect-ed warm electronics box. Thermal protection is provided by a nearly weightless material called silica aerogel. Three radioisotope heater units (RHUs) -- each about the size of a flashlight battery -- contain small amounts of plutonium-238 which gives off heat to keep the rover’s elec-tronics warm.
The rover’s wheels and suspension use a rocker-bogie system that is unique in that it does not use springs. Rather, its joints bend and conform to the contour of the ground, providing the greatest degree of stability for traversing rocky, uneven surfaces. A six-wheel chassis was chosen over a four-wheel design because it provides greater stability. For instance, one side of Sojourner could tip as much as 60 degrees as it climbed over a rock without tipping over. The wheels are 13 centimeters (5 inches) in diameter and made of stainless steel foil. Cleats on the wheels provide traction and each wheel can move up and down independently of all the others. Three motion sensors along Sojourner's frame can detect excessive tilt and stop the rover before it gets dangerously close to tipping over. Sojourner is capable of scaling a boulder on Mars that is more than 25 centimeters (10 inches) high and keep on going.
The rover will also perform a number of technology experiments designed to provide information that will improve future planetary rovers. These experiments include: terrain geometry reconstruction from lander/rover imaging; basic soil mechanics by imaging wheel tracks and wheel sinkage; and dead reckoning, path reconstruction and vision sensor performance of the rover.
In addition, Sojourner experiments will also determine vehicle performance; rover thermal conditions; effectiveness of the radio link; and material abrasion by sensing the wear on different thicknesses of paint on a rover wheel.
Scientists will study adherence of Martian crustal material by measuring dust accumulation on a reference solar cell that has a removable cover, and by directly measuring the mass of accumulated dust on a quartz crystal microbalance sensor.
The rover’s control system calls for the human operator to choose targets and for the rover to autonomously control how it reaches the targets and performs tasks. The onboard control system is built around an Intel 80C85 processor, selected for its low cost and resistance to single-event upsets from certain types of radiation. It is an 8-bit processor which runs at about 100,000 instructions per second.
Sojourner also carries an alpha proton x-ray spectrometer which must be in contact with rocks or soil to measure the chemical composition of the material being studied.
