Back in 2009, a defunct Russian satellite (Cosmos 2251) collided with another satellite (Iridium 33) belonging to a satellite phone network, spraying a field of debris across their two orbits and bringing the problem of space junk to public attention. Then on 13 January 2012, the International Space Station (ISS) was forced to fire the engines on its Svezda Service Module to manoeuvre the 450 tonne structure out of harm’s way, when ground tracking stations sounded the alarm that it was on a collision course with a piece of debris from the earlier smash.
The piece of debris in question was only about 10 cm across, so one might wonder what damage it could possibly do to something as big and solid as the ISS. But remember that the ISS orbits Earth at about 27,500 km/h, which gives us a rough idea of what the impact speed could have been. Compare this to a .50 calibre rifle bullet (the sort originally designed to let snipers disable parked vehicles and aircraft from a safe distance with a single shot) which is only 1.25cm’s across and leaves the barrel at a measly 3000km/h, and you’ll get an idea of the utter devastation that was avoided!
Luckily, the avoidance manoeuvre occured shortly before the ISS was scheduled to perform an orbital boost, so the two procedures were simply combined into one single operation.
Updated to include the names of the satellites involved in the earlier collision