Close Encounters of the Trash Kind

Every so often a piece of space junk comes dangerously close to the International Space Station. When that happens, the ISS crew takes shelter in the stations' two escape pods, waiting until the debris zips past and Mission Control gives the "all clear."

A tiny fleck of paint created this crater on Challenger's windshield.
Source: Universe Today.

The last time that happened was this past March, when Mission Control woke the sleeping ISS crew to send them scrambling for the escape pods. A piece of a smashed Russian communications satellite, Kosmos 2251, passed within nine miles of the ISS. That doesn't sound particularly close, but apparently the small size and erratic orbit of the Kosmos debris made it hard to track, necessitating caution.

The Soyuz TMA-7 spacecraft, similar to the ISS's Soyuz escape pods.
Source: Wikipedia. 

It's not a big surprise that a piece of Kosmos 2251 nearly crossed paths with the ISS. Back in 2009, that particular satellite accomplished a dubious "first" in space travel: it was the first man-made satellite to collide with another man-made satellite. Kosmos 2251 had been out of service for over a decade when its degrading orbit placed it directly in the path of the U.S.'s Iridium 33 communications satellite.

When they impacted, Kosmos weighed over 2,000 pounds; Iridium weighed over 1,200 pounds. Their relative speed was over 26,000 miles per hour! While Iridium 33 was a functioning satellite at the time of the impact,  it (obviously...) stopped working after getting smashed to bits by Kosmos 2251.

Iridium (Green) and Kosmos (Red), colliding over Northern Siberia.
Source: Celestrak.com.

This lead me to another question: did we know this crash was coming? As in, could it have been avoided?

The company Analytical Graphics runs a satellite tracking website analyzing space junk collision risks. On February 10, 2009, the day of the collision, AG's Celestrak website calculated that the two craft would have a close encounter, passing within 1,916 feet of each other. But this close encounter was not even predicted to be the nearest miss of the day. Therefore, it wasn't deemed worth the expense and expenditure of limited fuel resources to nudge Iridium a little further out of the way.

Kosmos 2251, back when it was in one piece.
Source: abovetopsecret.com.

I guess would have become clear that the 1,916 feet calculation was a bit off when Iridium (and some people's satellite phones...) stopped working at the moment the pass was predicted to occur!

Sources: Celestrak; Wikipedia; NASA; Space.com; NASA Television.