NASA recently wrote that the International Space Station has, during its first 15 years of operations, made 16 space junk collision avoidance maneuvers and been closely threatened another four times.

After a record number of four collision threats in 2012, no ISS collision avoidance maneuvers were required during 2013, reflecting the chaotic nature of the satellite population, said NASA's Orbital Debris Program Office in its quarterly newsletter.

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"In addition, three incidents arose when insufficient time permitted a collision avoidance maneuver, forcing the crew of the ISS to retreat to the Soyuz return craft during the time of closest approach, where they were prepared to undock from the ISS quickly in the event of a collision. In total, the collision avoidance maneuver threshold level has been reached only 20 times for an average of once per year."

That seems like a lot but considering the amount of debris that crosses its path in the course of a year its pretty amazing there have not been more close calls.

"The number of known objects that routinely transit the ISS orbital altitude is significant. In October 2013, the number of cataloged objects that posed potential threats to the ISS was in excess of 800, representing an increase of 60% from the population of November 1998 in an altitude region of ~415-420 km. Of these, 10% were spacecraft (operational and non-functional), a third were rocket bodies, and the remainder were miscellaneous debris. Although the individual masses of these objects varied from less than a kilogram to several metric tons, each was capable of inflicting serious damage to the ISS in the event of a collision," NASA stated.

NASA went on to note that space objects in circular orbits intersecting the ISS altitude normally drop below the ISS orbit very quickly, in days or weeks, but they can cross the space station's orbital plane up to 30 times per day.

In addition to the cataloged satellites, the U.S. Space Surveillance Network (SSN) maintains orbits on a large number (on the order of 5,000 or more) of objects that have not yet been officially cataloged. For October 2013, the number of such objects with orbits passing through the ISS altitude regime exceeded 200.

So what exactly triggers a collision avoidance maneuver? NASA says "if a predicted conjunction yields a probability of collision greater than 1 in 10,000, official flight rules call for the execution of a collision avoidance maneuver by the ISS unless such a maneuver would lead to an even greater risk to the ISS or its crew. "

See more here:

NASA: Space station dodged debris 16 times in past 15 years