Major contributors to space debris include the explosion of upper stages and satellite collisions.
[2] By 2015, the total had grown to 250 on-orbit fragmentation events.
[1] Also, about sixteen old Soviet nuclear space reactors are known to have released an estimated 100,000 NaK liquid metal coolant droplets 800–900 km up,[5] which range in size from 1 – 6 cm.
[5] The greatest risk to space missions is from untracked debris between 1 and 10 cm in size.
[1] Large pieces can be tracked and avoided, and impact from smaller pieces are usually survivable.