What is conjunction analysis?
Conjunction analysis is the systematic process of comparing the predicted future trajectories of all tracked space objects — active satellites, rocket bodies, and debris — to identify pairs of objects that will approach within a defined threshold distance. When the predicted miss distance and its associated uncertainty imply a collision probability above the operator's action threshold (typically 1-in-10,000 to 1-in-1,000), a Conjunction Data Message (CDM) is generated and the satellite operator is notified.
The US 18th Space Control Squadron
The primary provider of conjunction analysis globally is the US Space Force's 18th Space Control Squadron (18 SPCS) at Vandenberg Space Force Base, California. The 18 SPCS screens approximately 20 million satellite pairs per day against the full Space Object Catalog (~45,000 objects) and issues CDMs to registered satellite operators via the Space-Track.org portal. This service is provided free to all operators, commercial and governmental, worldwide — a de facto public safety service for the entire space industry.
Probability thresholds and manoeuvre decisions
When conjunction probability exceeds 1-in-10,000 (10⁻⁴), operators are expected to evaluate whether to manoeuvre. Above 1-in-1,000 (10⁻³), manoeuvre is generally mandatory under leading debris mitigation guidelines. The decision involves trade-offs: a manoeuvre consumes propellant (reducing satellite life), introduces new conjunction risks on the new trajectory, and may disrupt service. Starlink's automated collision avoidance system processes conjunction data and autonomously commands manoeuvres without human review — necessary at the scale of 10,000+ satellites.