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Pathfinder Rover Being Assembled

Sojourner is the name given to the first robotic roving vehicle sent to the planet Mars. Sojourner weighs 11.0 kg (24.3 lbs.) on earth (about 9 lbs. on Mars) and is about the size of a bag of groceries. This Microrover has six wheels and can move at speeds up to 0.6 meters (1.9 feet) per minute. This isn't very fast, but during the course of a day on Mars the Microrover can cover up to 3 meters. That speed is fast enough to accomplish many tasks near the lander.

The rover's wheels and suspension use a rocker-bogie system that is unique in that it does not use springs. Rather, its joints rotate 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 and obstacle-crossing capability. Six-wheeled vehicles can overcome obstacles three times larger than those crossable by four-wheeled vehicles.

For example, one side of Sojourner could tip as much as 45 degrees as it climbed over a rock without tipping over. The wheels are 13 centimeters (5 inches) in diameter and made of aluminum. Stainless steel treads and 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 20 centimeters (8 inches) high.