Named in honor of Thomas J. Ahrens, professor of geophysics at the California Institute of Technology. Ahrens has built and directed the premier laboratory for experimental impact and shock physics, using light-gas guns. He has used this laboratory to investigate a broad array of problems, including the equations of state of minerals at very hight pressure, shock metamorphism and cratering mechanics. He has also addressed problems of impact cratering by computer code calculations and investigated the accumulation and early bombardment of the earth and the evolution of the atmosphere. (M 18645) _ _.