Monitoring a high risk conjunction between two large defunct objects in LEO
Here's my rough impression of what we may expect to see if the collision happens. Note that the collision isn't going to be perfectly heads-on, and is statistically unlikely for the centers of mass to collide.BEFORE:       ^    /
       |   |
       |   o - CZ-4C R/B
       |  /
       | |
       | /
       |/
       |
       *
      /|
     | |
     / |
    /  o - COSMOS 2004
   |   |
   /   |
  v    |

AFTER:     aaa^    /
     aaaa   |
      aaaa  |
       aa  /
     b aa b b
   bb b bb b b
       ccc  b b
      ccccc  b b
   b  ccccc  b b
 b b   ccc
  b b b b  b
   b  a |
     aa |
   aaa  |
   aaa  |
   aaa  |

Legend:a - Shattered fragments of the parts of satellites that weren't involved in direct hypervelocity collision, but got torn off and spun. Essentially like a shotgun discharge, but in space. These will carry most of their original kinetic energy and follow close to their satellite's original trajectory, slowly spreading over time and becoming untrackable. These will stay around the longest.b - Pieces that were close to or directly involved in the collision, retaining enough orbital velocity to stay in space. May be on highly eccentric orbits. Should decay faster than pieces labeled "a" (I think, I'm not sure).c - Pieces that lost enough velocity to fall either straight down, or fly upwards and then fall straight down. These may cause some damage on their way down to ground, but should disappear quickly.For reference, here are some (much prettier) diagrams of the 2009 collision, that happened at close to 90 degree angle: http://celestrak.com/events/collision/.