Goddard Project

Explosions of upper stages from old satellite rockets are of particular concern because of the large volume of debris they generate. There have been more than 190 such explosions to date.

Also of concern are obsolete satellites that remain in key orbits where they contribute to overcrowding and increase the likelihood of collision. In February 2009, a U.S. communications satellite, owned by Iridium Satellite LLC, collided with an inactive Russian military communications satellite at a closing speed of 22,000 miles per hour. Both spacecraft were destroyed and the collision created nearly 900 pieces of trackable debris.

Nicholas Johnson at NASA reports some orbits are so cluttered already that a chain reaction caused by colliding debris is virtually inevitable, rendering some orbits inaccessible for millennia.