Satellites may fail during their operational life, causing ground communication with them to be degraded or severed entirely.
A lost satellite continues to orbit aimlessly in space and presents a risk of collision for other objects. Failures most often occur during the first few months a satellite is in space.
Source: AXA XL
The risk of failure presents an important challenge for satellite operators. Although redundancy is thoroughly embedded in the design of a satellite, and other measures are implemented to ensure satellites remove themselves from orbit if their systems fail, a 100% success rate cannot be guaranteed in space.
Today there is an estimated 3,000 unresponsive satellites in space.