Increasingly sophisticated technologies are needed for counterterrorism responses to biological and chemical warfare agents. Recently developed detection and identification systems are characterized by increased sensitivity, greater automation, and fewer false alarms. Attempts are also under way to reduce the cost and complexity of field-deployable systems. A broad range of decontamination reagents for equipment and personnel is emerging, but decontamination of large buildings, inaccessible spaces, and sensitive equipment remains problematic.

The largest source of error in the deduction of atmospheric densities from orbital data stems from uncertainty in our knowledge of the interaction of atmospheric molecules with satellite surfaces. A method of determining two of the important parameters, the accommodation and drag coefficients, is developed in detail. As an example, the method is applied to observational data from the paddle wheel satellite Ariel 2. Because of the importance of spherical sateilk{es the drag coefficient for a sphere under the conditions of Ariel 2 ( a moderately eccentric orbit with perigee near 300 km) is also obtained. The importance of well-designed orbital experiments to improve our knowledge of surface-particle interactions at a variety of altitudes is stressed.2 and derived an accommodation coefficient from observa-parameters and surface conditions. The Schamberg model was developed to describe the interaction of artificial satellites with the upper atmosphere. The appropriate aerodynamic regime is 'free molecular' flow; i.e., the effects on the satellite can be deduced by considering only 3077

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