A one-centimetre fragment in LEO carries the energy of a bullet; a few centimetres can disable a satellite. The higher the orbit, the longer debris stays. A chain reaction in which collisions create ever more debris (the Kessler syndrome) could make certain orbits unusable for years — including for whoever caused the debris.
Key point
Space debris makes kinetic ASAT weapons a double-edged sword. That explains why great powers invest mainly in reversible means and in resilience (many small satellites, see 2.8), and why destructive tests are increasingly condemned internationally.
How crowded the orbits already are is shown by the Orbital Radar debris map and its statistics (see 10.6).