This paper described an experiment to determine the rate of decay of microwave backscattering from a controlled cesium plasma in the wake of spheres entering the earth's atmosphere at hypersonic velocity. The maximum 1/e decay time observed was approximately 0.1 s. The scattering decay function found was accurately exponential over 5 orders of magnitude in intensity. The observaed decay rate of the plasma is interpreted in terms of a simple model using thre-body electron attachment to molecular oxygen and an effective diffusion constant D', which varies as a function of time after the plasma is produced. Values of D' obtained at between 45- and 50-km altitude ranged between 2¿102 and 3¿103 cm2/s. The asymptotic value of D' obtained in the analysis for late times approaches the value of the mutual diffusion coefficient for the ambient air at that altitude. The results obtained indicate that there is little turbulence in the atmosphere at about 50-km altitude, at least as it corresponds to the scale size measured in the experiment.