"After a few days on the Martian surface, when we left the rock named Yogi, we turned on Sojourner's hazard avoidance system and asked it to start making some of its own decisions," said Brian Wilcox, supervisor of the robotic vehicles group at JPL. "This hazard avoidance system sets the rover apart from all other machines that have explored space. Sojourner had to make that trip to the next rock without the benefit of detailed information to warn it of obstacles along the way."

Sojourner's intelligence is the product of many years of research in the fields of telerobotics and automation. Beyond the challenges of building a robot that can survive the frigid climate of another planet, Sojourner is taking the robotic exploration program one step further to a day when truly "smart" machines will be able to explore hundreds of kilometers away from a landing site all on their own.

"Because Sojourner is our first rover, we designed it to be quite safe," Wilcox said. "She moves slowly (0.7 centimeter per second or 0.3 inches per second) and stops a lot along the way to sense the terrain and process information. Her IQ is probably not as high as an insect's, but to put it in the proper perspective, consider that even a house fly has more computing capacity than the largest supercomputers today."

Sojourner's hazard avoidance system is comprised of five laser stripes that project out to the ground. One laser is located in the center of the rover body and points straight ahead. Two of the laser beams are pointed outward at a modest angle from the center beam, while two more project much farther outward, like periphery vision, beyond the rover's body.

"These lasers cut across anything in the rover's path," Wilcox said. "By looking at the shape of the stripe with the rovers cameras, we can detect rocks and build contour maps of the terrain immediately in front of the rover. The rover then moves three inches ahead and projects its laser beams on the ground again. We take that data and continue to build our terrain maps based on the shapes of the laser beams as they bend with the shape of the ground."

To complement its laser eyes, the rover has three levels of autonomy to use in choosing its path across the Martian sand. The first level involves little risk: it instructs the rover simply to avoid all but the flatest ground. The higher levels of risk cause the rover to drive over bigger and bigger obstacles without trying to avoid them, with the highest risk level allowing the rover to climb over rocks almost as big as the wheels.

"If we were exploring a dry riverbed, which would be relatively smooth and devoid of rocks, the first level of risk would probably be all we needed to drive a rover," Wilcox said. "But given the terrain we've landed in, which contains a huge variety of rocks, we have to use the higher levels of risk. Just like on the freeway, you would always drive around any small object if you could. In more rugged terrain, we don't have that luxury."

Another type of autonomy command tells the rover to find rocks.

"We tested this mode when Sojourner made its way to the rock named Souffle," Wilcox said. "We gave it only the x- and y-axis coordinates, then told it to find a rock. It used its laser hazard sensor to find, instead of avoid, the rock. If the coordinates were about right, then it knew it must be at the right rock. And even though the wheel slippage and gyro drift would have caused the rover to be more than 30 centimeters (1 foot) away from Souffle, the rover instead parked itself within 4 centimeters (2 inches) of the spot we wanted near the rock."

Sojourner's intelligence is based on a mathematical model that emulates animal behavior. The ability for animals to avoid or flee danger, such as predators, is a low level reflex, Wilcox explained, but it gets the job done. "The rover can distinguish between obstacles that pose no threat, such as a two- or three-inch-tall rock that can be driven over, as opposed to a 10- or 12-inch-tall rock that might tip it over," he said. "As soon as the rover recognizes the hazardous rocks, it switches into the hazard avoidance behavior and begins to turn until it no longer sees the obstacle. Then it moves forward and returns to a course toward the current goal."

Sojourner's reasoning capabilities deviate from the strict sequence of instructions used on prior space explorers. Previous spacecraft have operated solely using sequences of instructions created by human operators on the ground, along with preprogrammed "safing" sequences which protect the spacecraft from harm and re-establish contact with Earth if some failure should occur. Sojourner has both of these capabilities, but also has reflexes to avoid hazards and respond to unplanned events. This is necessary because it would go much too slow if humans on Earth had to make every decision when interacting with the unknown environment.

"This is what allows Sojourner to take a stroll on its own," Wilcox said. "If it encounters new rocks that it had not anticipated, the rover will use sensory information provided by the lasers to circumvent the obstacle. If the wheels on one side slip in the dirt, it will begin to turn until it is going back in the right direction."

Sojourner's major limitations lie in its dependence on the Pathfinder lander for communications. If it journeys much beyond the lander's line of sight, it will become marooned. Already on the drawing boards for future missions are rovers that will be able to communicate with orbiters flying overhead a landing site. Engineers hope to gain a much better understanding of Sojourner's interactions in the Martian environment to make this concept a reality.

"We are already learning a lot about the demands of driving a rover," Wilcox said. "Right now we're developing a brand new command sequence for the rover every day, but we're accomplishing that with significantly fewer people than we did in the Voyager days. In those days, it took as many as 700 people to fly the spacecraft, whereas Sojourner only requires about seven people."

Powerful visualization tools not yet invented 20 years ago have also advanced Sojourner's and future rovers' autonomous capabilities. For instance, with a specially designed computer program, stereo images taken by the lander or Sojourner are integrated into a three-dimensional view of the rover's location. Engineers can see the rover's tire tracks and know where it has gone. Their view of the immediate surroundings also helps them decide where Sojourner should go next.

"In 10 to 20 years, I'd like to see a rover that could go anywhere on Mars," Wilcox said. "I'd like to launch a Lewis and Clark expedition, where we would basically travel 1,000 kilometers cross-country with our rover."

Even in the shorter term, though, the strides made by Sojourner's travels on Mars will be evident. The next rovers to explore the red planet will have at least 1,000 times the processing speed of Sojourner and will be able to travel much farther much faster.

"Tomorrow's rover will be able to plan and think ahead," Wilcox said. "Whereas Sojourner can only plan 20 centimeters ahead, our next rover will be able to plan 10 meters or more ahead. And while Sojourner has no concept of what rocks look like 20 meters away, her successor will be able to recognize rocks that far away very easily."

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