Complex sentence: Beyond a critical density, the Kessler syndrome becomes possible, a runaway chain reaction of collisions creating more fragments which exponentially increases the amount of debris, affecting useful polar-orbiting bands, increasing the cost of protection for missions and destroying live satellites.
Split into simpler sentences: Beyond a critical density, Kessler syndrome ensues--a runaway chain reaction of collisions exponentially increasing the amount of debris. This affects useful polar-orbiting bands, increases the cost of protection for missions and destroys live satellites.