"Between 775 and 975 kilometers overhead, derelict satellites pass within 1,000 meters of one another 1,000 times a year. Any collision would instantly double the amount of trackable debris in orbit and create countless smaller, yet still dangerous, bits of space junk to rain down on valuable satellites below them."
There Is Too Much Trash in Space
Debris from spacecraft threatens the burgeoning space economy. We need a global agreement to keep space clean
scientificamerican.com