WASHINGTON (AP) - Arthur G. Stephenson, director of NASA's Marshall Space Flight Center in Huntsville, Ala., will head the investigation into the failure of the Mars Climate Orbiter mission. The spacecraft was lost Sept. 23 as it entered orbit around Mars. A preliminary investigation indicated that the cause was failure to convert measurements from one system to another. One group of technicians was using the metric system, normally used in science worldwide, while another reportedly supplied measurements in the English system.