Nothing in low Earth orbit stays up forever. The thin upper atmosphere still exerts a faint drag, so satellites slowly lose energy and spiral lower in a process called orbital decay. Eventually they reenter and burn up; most small objects are destroyed by the heat, while occasional larger pieces survive to reach the surface. The ISS counters this drag with periodic reboosts.
Alongside the working satellites is a far larger population of space debris: spent rocket stages, dead satellites and fragments from past collisions and anti-satellite tests. Tens of thousands of pieces are large enough to be tracked, and far more are too small to catalog yet still travel fast enough to cripple a spacecraft on impact.
The nightmare scenario is the Kessler syndrome — a runaway chain reaction in which one collision throws off debris that triggers further collisions, each adding more fragments, until certain orbits become so littered they’re hazardous to use for generations. Even short of that extreme, every new fragment raises the collision risk for everything sharing its altitude.
Megaconstellations have sharply increased this “space traffic.” Networks of thousands of satellites bring fast global internet but crowd popular LEO altitudes, force frequent collision-avoidance maneuvers, and brighten the night sky in ways that frustrate astronomers. Tracking the catalog — predicting close approaches and reentries — is how operators keep the orbital lanes usable.