SPACE WEATHER - AGI

Section I: Residual Atmosphere Effects© SSS Educational Series 2012Figure 3, drag coefficient, Cd, the velocity of the spacecraft relative to the ambientatmosphere, and the atmospheric density (which is a function of altitude). 3Figure 3: Orbital lifetime as a function of altitude (Source: NASA).Among these, the most important parameter is the atmospheric density which varieswith solar activity.The height of the top of the thermosphere (the thermopause) lies at about 500 kmduring solar minimum and climbs to about 1,000 km during solar maximum, as thethermosphere expands due to increased absorption of solar ultra-violet radiation.The heating and expansion of the thermosphere causes drag at higher altitudes.Many spacecraft orbit within the thermosphere (including the ISS and the spaceshuttle), hence this varying drag must be taken into account when calculating orpredicting orbits.Spacecraft in LEO experience much lower drag, and thus slower orbital decay,during solar minimum, and higher drag and faster decay during solar maximum. Inaddition, spacecraft with low ballistic coefficient (low mass-to-area ratio) will respondquickly to the atmosphere and decay relatively faster, while those with high ballisticcoefficients will survive a larger number of solar cycles and decay more slowly [1].Periods of high solar activity are accompanied by sudden perturbations in the orbitsof LEO which could lead to changes in the location of tracked objects (Figure 4).Abrupt changes in the solar activity (from geomagnetic storms, solar flares, etc.) cancause short-term changes in the atmospheric density and hence vary the drag onspacecraft. This can affect spacecraft’s predicted orbital lifetime and therefore the3 Generally, atmospheric density decreases with increasing altitude.3