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water once existed on Mars, they will have found an important indi- cator for the existence of past Martian life.

Like Sojourner, Spirit and Opportunity carry instruments to ex- amine rocks and soil, in the hope of finding detailed geological evi- dence for the past presence of water. However, the new rovers travel much faster than Sojourner did, covering in three Martian days the same 100 meters (330 feet) that Sojourner took 12 weeks to cover.An Earth-based controller can send a radio message to a rover telling it what to examine, but even at the speed of light, radio waves from Earth take minutes to reach Mars, making it impossible to drive the robot in real time.Thus an exploring rover is on its own and must see well enough to safely reach a specified site over rough terrain.

A rover does this by first determining its present location. It could do so by tracking every turn of its wheels since leaving its landing site, like an automobile odometer. However, wheels tend to slip on rocks and sand so instead the rover uses what Larry Matthies calls visual odometry.Seeing the world in three dimensions through two video cameras, as we do through our eyes, it maps the peaks and valleys, the rough and smooth areas of its neighborhood.Then it selects a promi- nent benchmark feature, perhaps a tall rock with a distinctive shape that it can recognize from varied distances and angles. Referring to this landmark, the unit can determine where it is to within 1 percent of the distance it has traveled.After establishing its location, the rover plans its trek to the target area. Like a human mountain climber scan- ning the terrain ahead for the best route, it examines its three-dimen- sional map to determine surface roughness, grade steepness, and obstacles, and selects the best path.

If all goes as planned, this version of autonomous robot vision will play a central part in a mission costing $800 million. NASA sees the current mission as a prelude to a 2009 one, where an even more capable rover will move to selected rocks, pick them up, and carry them back to a spacecraft that will return those pieces of Mars to Earth.

Selecting visual landmarks for navigation also works on Earth. Paolo Pirjanian, Chief Scientist of California-based Evolution Ro-

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