Before the Chinese A-Sat weapon tests, the #1 source of space junk was radioactive coolant leaking from old Soviet nuclear reactors in space (not RTGs, literally nuclear reactors)
"The Soviet Union has been flying nuclear power sources in space since about 1965. For the most part these nuclear power sources have been low-power nuclear reactors using a thermoelectric conversion principle."
The thing 'rebelgecko is alluding to is specific, and fascinating! The coolant in these space reactors is a molten metal (low-melting point sodium-potassium eutectic). When they leaked in the space vacuum, the liquid dispersed into a large number of small, metal spherules, which formed rings around Earth. Because the spherules are metallic and smooth, they're (comparatively) easy to track by radar, from Earth.
