Why a Satellite’s Orbit Can Slowly Decay
A satellite in low Earth orbit is moving sideways fast enough that, as it falls toward Earth, the planet’s surface curves away beneath it. Even the extremely thin upper atmosphere can disturb that balance. Molecules striking the spacecraft exert a tiny force opposite its motion relative to the air—a force called atmospheric drag.
Each encounter takes a little energy from the orbit. The satellite’s path gradually contracts, bringing it into regions where the atmosphere is denser. That can increase the drag and speed up the decline, though the rate depends on conditions in the upper atmosphere, which can expand or contract as solar activity changes.
The Orbit Becomes an Engineering Constraint
For a satellite that must stay in a particular orbit, operators can use onboard thrusters to restore some of the lost motion. That requires fuel, so drag helps determine how long the spacecraft can remain useful. In very low orbits, it can be a major limit; higher up, the atmosphere is thinner and the effect is generally weaker.
Drag also matters at the end of a mission. A spacecraft left in a low orbit will eventually descend far enough for the atmosphere to heat it strongly during reentry. Designers therefore account for orbital decay when planning a satellite’s lifetime and what happens after its work is done. A satellite may look as if it is coasting through empty space, but its orbit is continually negotiating with the faint air below it.
