By far the greatest tool for preventing collisions is careful monitoring. NASA and the Department of Defense currently track over 21,000 objects using ground- and space-based surveillance systems, following pieces as small as 2 inches in diameter.
This detection will soon be twice as sensitive; the U.S. Air Force "pathfinder" satellite expected to launch this year
will be the first-ever satellite devoted solely to tracking debris. It should help fill in holes in the data, and hopefully prevent collisions like the much-publicized crash
of two satellites in 2008.
The Long Duration Exposure Facility (LDEF) satellite, shown here docking above the Space Shuttle Challenger, was another important asset, having tracked debris for six years before being decommissioned in 1990.