Using data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer aboard NASA's Terra satellite, geologists at Arizona State University in Tempe are learning why cities form the way they do. An analysis of 12 cities from nine countries already has revealed that there are three classes of cities -- decentralized, centralized and intermediate -- determined by density, growth pattern, and presence or absence of an urban center. The instrument aboard Terra produces images at 14 wavelengths extending from the visible through the infrared, and it has a resolution of 15 to 90 m2 per pixel. The researchers hope that a study of 100 cities worldwide will define issues in urban ecology, including the influence of geography and social factors on city dynamics.