Optimal Path Planning of Multiple Mobile Robots for Sample Collection on a Planetary Surface

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In the exploration of a planetary surface such as that of Mars using mobile robots, rock and soil-sample collection and analysis are essential in determining the terrain composition and in searching for traces of ancient life (Malin & Edgett, 2000). Several missions to Mars have already been sent. In the 1997 Mars Pathfinder mission (Mars Pathfinder Homepage), the Sojourner rover used an alpha-proton-X-ray spectrometer (APXS) to analyze rock and soil sample compositions. It also had a simple onboard control system for hazard avoidance, although the rover was operated remotely from Earth most of the time. The method for rock and soil-sample collection is as follows. After landing, the rover used its black-and-white and color imaging systems to survey the surrounding terrain. The images were sent back to Earth, and analyzed by a team of geologists to determine where interesting samples might be found. Based on that information, the next destination for the rover was selected and the commands to get there were sent to the rover via radio with transmission delays ranging from 10 to 15 minutes (depending on the relative orbital positions of Earth and Mars). The set of commands were sent out over a day with the rover moving only a small distance each time. This was done to allow the mission control to constantly verify the position, with time to react to unforeseen problems. When the rover finally reached its destination and analyzed the sample, it spent another day transmitting the information back to Earth. The cycle was repeated as soon as the geologists had decided on the next destination for the rover. Clearly, an automated system for rock and soil sample collection would expedite the process. In the 2004 Mars Exploration Rover (MER) mission, the Spirit and Opportunity rovers (Mars Spirit & Opportunity Rovers Homepage) featured an upgraded navigation system. Imagery from a stereo camera pair was used to create a 3-D model of the surrounding terrain, from which a traversability map could be generated. This feature gave the mission controllers the option of either directly commanding the rovers or