Atmospheric Drag and Orbital Decay

Space is not perfectly empty. In Low Earth Orbit, the thermosphere exerts a constant drag force on spacecraft.

Below 600km, atmospheric drag is a significant factor. Satellites must carry propellant to perform periodic re-boost maneuvers, or their orbits will decay until they burn up in the atmosphere.

Solar Max

The density of the upper atmosphere is highly dependent on solar activity. During the solar maximum of the 11-year cycle, the atmosphere expands outward due to increased ultraviolet radiation heating, drastically increasing drag on LEO satellites.