Debris
Debris refers to space vehicle components left behind from decoupling of tanks or engines, or the parts not destroyed in a catastrophic failure. These parts remain behind, and when in space, orbits continue to be tracked for these objects.
The user can track debris from the Tracking Station, debris tab.
If the object has landed on Kerbin, it can be recovered via the Tracking Station.
If the object is in space or on another celestial body, it can be terminated via the Tracking station.
If an object contains stranded Kerbals, termination kills the crew and adds them to the lost list in the Astronaut Complex.
If the object happens to be a discarded rocket stage that still has fuel, and a docking port, but no Command module, the object can be tracked and the fuel recovered by docking and executing a fuel transfer.
The default quantity of tracked objects is set to 250, but can be changed in the Settings menu.