Massive Derelict Watch

• Massive Derelict: Either an abandoned rocket body or a non-operational payload, with a mass of 700 kg or more. These are of greatest concern since they have no ability to avoid collisions and, if they were to collide, they would produce many thousands of debris fragments. 

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• Lethal Non-Trackable (LNT): An LNT is a piece of debris in LEO that is large enough to cause mission-terminating damage to an operational  satellite (> 5mm) but too small to be detectable by radar (< 10cm). LNTs make up 98% of the  LEO debris population as the cataloged population in LEO is ~20,000 and there is an estimated ~600,000 to 900,000 LNT in LEO. 

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• Conjunction: A conjunction is an instance when two derelicts pass less than 5km from each other; also known as an “encounter.”

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• Cluster: A cluster is a collection of massive derelict objects that reside in a narrow altitude range. Characterized by frequent conjunctions and a statistically higher probability of collision than average in LEO.

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• Pure Cluster: A pure cluster is a cluster of Soviet/Russian paired rocket bodies and non-operational payloads (“paired” meaning the payload plus the rocket that deployed it). Consists of 490 Soviet/Russian rocket bodies and non-operational payloads. The four pure clusters are                   ,           ,           , and              (denoting the altitudes at which their orbits are centered). These clusters have been monitored since May 2016. 

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• Complete Cluster: Complete clusters are groups of massive derelicts (i.e., mass greater than 700 kg) in LEO (i.e., an altitude below 2,000 km) that reside in altitude bands that are identified by the approximate center of the cluster. The complete clusters are           ,            ,            ,            ,             , and              that comprise 1,269 objects from many nations. The complete clusters were identified and started being tracked beginning 1 October 2019. Massive derelicts in LEO that span more than one cluster are included as the complete cluster “          .” Likewise, massive derelicts in elliptical orbits with a perigee below 2,000 km and an apogee near semi-synchronous or geosynchronous orbits are included as the complete cluster “            .” 

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• Risk: Risk is the product of the probability of an event occurring between two cluster members and the consequence of that event. The surrogate for probability of a collision is derived from the spatial density of the objects within the clusters. Similarly, the consequence is derived from the total mass involved in a collision; this is proportional to the number of fragments that will likely be produced as the result of a collision.