Strictly speaking, "space junk" is composed of spent rocket casings, malfunctioning satellites, & other man-made trash (for instance, the toolbag that was dropped on the last Shuttle mission). That is, indeed a problem (see the NASA HQ library resource list on the subject at http://www.hq.nasa.gov/office/hqlibrary/pathfinders/debris.htm). 

I think you're thinking of near-Earth objects-comets & asteroids that cross our Earth's orbit & may collide with it. That's also a problem, of course (see the NASA HQ library resource list on the subject at http://www.hq.nasa.gov/office/hqlibrary/pathfinders/aster.htm).