How It Works
Even at orbital altitudes, Earth's atmosphere doesn't end abruptly. Trace amounts of gas extend hundreds of kilometres into space, creating a thin but persistent drag force on orbiting objects. This drag slowly removes energy from the orbit, lowering the satellite's altitude. As the object descends into denser atmosphere, drag increases, accelerating the process until re-entry.
Factors Affecting Decay Rate
Altitude: Below 400 km, decay is measured in months. At 600 km, it takes years. At 800 km, decades to centuries. Above 1,000 km, objects remain for millennia. Area-to-mass ratio: Large, lightweight objects (like solar panels or thermal blankets) experience more drag relative to their mass. Solar activity: An active sun heats and expands the upper atmosphere, dramatically increasing drag. During solar maximum, satellites at 400–600 km can lose altitude several times faster than during solar minimum.
Why It Matters for Debris
Orbital decay is Earth's natural debris cleanup mechanism — but it only works in LEO. Debris at 700–1,000 km decays over decades to centuries, persisting long enough to pose collision risks. This is why Kessler syndrome concerns focus on these altitudes. SpaceX chose to lower Starlink's operational altitude to ~480 km partly to ensure faster natural decay of any failed satellites.