FIGURE 2.1 Atmospheric structure and processes. SOURCE: Adapted from J.T. Emmert, A physicist’s tour of the upper atmosphere, Physics Today 61(12):70-71, 2008.


Atmospheric drag is the largest source of uncertainty in orbit determination and prediction for low-perigee objects. Most of the uncertainty stems from inaccurate knowledge of atmospheric density (the remainder is due to inaccurate modeling, discussed in the next section, of the interaction between the atmosphere and an object).4 On global scales, for example, the root-mean-square relative error of density models generally decreases with decreasing altitude (from 20-25 percent at 600 km to 5-10 percent at 200 km). However, the drag force increases exponentially with decreasing altitude, and atmospheric variations on smaller spatial scales become increasingly important, which presents enormous challenges for accurate reentry predictions.

Earth’s thermosphere (~90-600 km altitude) and exosphere (>600 km) are a hot, partially ionized gas. This region is heated primarily by absorption of solar ultraviolet radiation, by electromagnetic energy and energetic particles from the solar wind, and by dissipation of waves propagating upward from the underlying atmosphere (see Figure 2.1). The heating is balanced by cooling from infrared emissions in the lower thermosphere, primarily by the trace species CO2 and NO. Cooling is very inefficient in the upper thermosphere, which sheds its heat via thermal conduction to the cooler lower thermosphere. Below ~100 km, the atmosphere is well mixed (~78 percent


4 M.F. Storz, B.R. Bowman, M.J.I. Branson, S.J. Casali, W.K. Tobiska, High accuracy satellite drag model (HASDM), Advances in Space Research 36:2497-2505, 2005.

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