Wheels: Mars Pathfinder Rover
Talk about a cool set of wheels: This baby had six of them, each powered by its own independent motor. But it wasn't built for speed; it was built to handle terrain that has given new meaning to the word "rough." Meet Sojourner, Pathfinder's solar-powered go cart.
The rover was named Sojourner by 12-year-old Valerie Ambroise, from Bridgeport, Connecticut, who won a worldwide contest to dub the little buggy. The word sojourner means "traveler," but Valerie was also thinking of Sojourner Truth, a former slave, abolitionist, and champion of women's rights.
Sojourner was only 63 centimeters (25 inches) long -- about the size of a microwave oven. On Earth its weight was 25 pounds, but thanks to the lower gravity on Mars, it weighed in at only about 10 pounds on the Red Planet.
The rover was ground tested at JPL on a specially designed "testbed" that simulated the Martian terrain. It made its first solo run at the mouth of Ares Vallis, a dry river on Mars, shortly after touchdown on July 4,
when it drove down a ramp from its mother ship onto the Martian surface.
With an average speed of 6 millimeters (0.25 inch) per second, Sojourner used an onboard control system to creep forward, back up, and redirect itself in a circuit around its home base, the Pathfinder lander that ferried it from Earth to Mars. To return
the favor, Sojourner took black-and-white snapshots of the lander and transmited them back to Earth so mission scientists could check the condition of the lander for any damage it may have suffered during entry and landing.
Sojourner's laser navigation system could sense the distance of obstacles in its path. Its six wheels, arranged without axles in a "rocker-bogie" design provided maximum maneuverability, allowing the rover to climb over obstacles half its height. In addition to the lasers, the rover carried a black-and-white stereo camera to create "virtual reality" scenes of its travels. It also had a color camera for close-ups of rocks and soil. Its most important tool was an alpha proton X-ray spectrometer which could "sniff" the atomic makeup of the rocks it encounters, allowing scientists to identify the mineral types on Mars.
No need to worry about miles per gallon. Sojourner's roof was covered with solar panels, so it drew its fuel from the sun. For that reason, though, hours of operation were limited to the four hours surrounding high noon on Mars, with lithium batteries providing backup.
Sojourner was semi-autonomous. Many of its movements were controlled from Earth, but because there was a time delay of more than 6 minutes, depending on the relative position of Earth and Mars, Sojourner was designed to make some decisions on its own. An Intel 8-bit processor that could churn out 100,000 instructions per second kept it on target.
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