The U.S. Space Surveillance Network (SSN) is a collection of radar and optical sensors used to detect, track and identify objects in space. Although referred to as a network, the Space Surveillance Network was not originally planned as such. As various sensors became available, their particular capabilities were used to contribute to the space surveillance mission. The Space Surveillance Network cannot continuously track all satellites, therefore the Space Surveillance Center prepares a prioritized list of satellites to track. Generally, satellites with high interest missions or unstable orbits (objects about to deorbit) will have higher priority data collection requirements than other satellites.